By Publicis Groupe told clients this month to stop using The Trade Desk. The Trade Desk's CEO fired back on LinkedIn, accusing agencies of running from transparency while arbitraging programmatic. The press releases used words like "partnership values" and "audit rights."
Here's what actually happened: the holding companies have spent nearly a decade rebuilding the exact rebate economics that the ANA exposed in 2016, and the new packaging is finally tearing at the seams.
The billion-dollar memo nobody was supposed to see
In November 2025, a former WPP executive named Richard Foster filed a wrongful termination lawsuit seeking more than $100 million in damages. Foster, who served as CEO of Motion Content Group, alleged he was fired after raising objections to WPP's media rebate practices. That's a story in itself.
But the real gift came in WPP's counter-filing, which made public a 2024 memo Foster had authored. According to the memo, GroupM exceeded $1 billion in global net sales for what WPP internally calls "non-product related income" in 2023. That's WPP's sanitized label for principal-based media revenue. The memo also referred to the practice as "proprietary media trading" and "purchase risk media deals," depending on which internal audience needed convincing.
The memo projected that revenue line would grow 15% in 2024, even while WPP's overall net revenue was slightly declining. Let that land for a second: the fastest-growing revenue stream inside one of the world's largest agency holding companies is money made by buying media wholesale and marking it up to clients.
Same game, different decade
If this sounds familiar, it should. In June 2016, the Association of National Advertisers and auditing firm K2 Intelligence published a report documenting pervasive cash rebate practices across US media buying. Agencies were collecting kickbacks from media sellers based on spend volume, and those refunds never made it back to clients. The industry collectively gasped, promised reform, and then quietly moved on.
Direct rebates are still technically prohibited in the US, same as they were a decade ago. They remain commonplace in Europe, where nobody pretends otherwise. But American holding companies found a workaround: principal-based trading. The agency buys inventory on its own balance sheet at wholesale rates, marks it up, sells it to clients, and pockets the spread. It's not a rebate. It's a "media investment product."
The incentive structure is identical to what K2 documented. The money flows the same direction. The only thing that changed is the paperwork.
The numbers everyone pretends not to know
Here's where following the money gets interesting.
WPP's principal media revenue crossed $1 billion in 2023, according to Foster's memo. We only know this because of a lawsuit. WPP does not voluntarily disclose the figure.
Omnicom reported $4.1 billion in "third-party service costs" for full-year 2025, per its earnings release. During Publicis's earnings call, its CFO noted that principal media accounted for slightly more than half of the company's third-party service costs. If Omnicom's mix is even roughly comparable, its principal media revenue could be north of $2 billion.
Meanwhile, Publicis CEO Arthur Sadoun told a Barclays analyst last month that principal media "is not a strategic priority for us" and that Publicis is "not the biggest player by far." Publicis does not disclose its principal media revenue either. Of course not.
Omnicom CEO John Wren was more candid during the company's Q2 2025 call: "It's a product we've had for a long time, a product that continues to grow, and I can see very clearly that it's going to continue to grow into the future." Then he added, regarding competitors: "Everybody else that you speak to in the industry doesn't tell you the truth."
He's not wrong about that.
The Trade Desk picks a fight
The Trade Desk has historically positioned itself as the agency's best friend in programmatic. That posture has limits, apparently.
After Publicis told clients to drop TTD following an audit dispute, CEO Jeff Green posted on LinkedIn calling out agencies "who wave the flag of transparency publicly, but run from it in practice as they arbitrage in the inefficiencies of programmatic." He specifically named principal-based media as the arbitrage mechanism.
During TTD's full-year 2025 earnings call, Green told investors he doesn't think agencies are "doing as good of a job representing their clients as they could." When asked about WPP and Dentsu pulling back from OpenPath, TTD's direct-to-publisher integration, Green said: "I'm not surprised that some people would like to turn that off if they find ways to get paid in unique ways."
"Unique ways" is doing a lot of heavy lifting in that sentence.
Who benefits, and who's asking the wrong questions
The ANA's current position is measured. Group EVP Bill Duggan told AdExchanger that marketers need to "take a proactive stance in educating themselves on the benefits and challenges of principal media" and called it "buyer beware" without clear contractual guardrails.
That framing treats this as an informed-consent problem. It's not. It's an incentive-alignment problem.
When your agency makes its highest margins by steering your spend toward inventory it already owns, the agency is no longer acting as your agent. It is a counterparty. WPP's own legal filings revealed that none of its top 20 clients participate in principal media. Several of those clients, including Paramount, Comcast, Amazon, and Google, are themselves supply-side purveyors of principal media deals. They know exactly how the economics work, which is precisely why they don't buy through it.
The clients getting steered into principal media inventory are the ones without the sophistication or the leverage to say no. That's been the pattern since the rebate era, and it hasn't changed.
What comes next
GroupM CEO Brian Lesser told WPP investors last month that clients navigating addressable TV, social, and retail media are under pressure to prove performance, and that principal media offers a cheaper route. "They are asking us for more, frankly," Lesser said.
Don't expect this to slow down. Agency holdco investors aren't concerned about principal media. They're asking for more of it. It produces some of the highest-margin revenue these companies report. The bankers want growth, and this line delivers.
Foster's lawsuit proposed alternatives in his 2024 memo, including upfront investments in brand-backed content for streaming platforms and ad credit deals with publishers like A+E, TikTok, and Roku. But those require creativity and relationship-building. Principal media just requires a balance sheet and clients who don't read the fine print.
The mechanics will keep morphing. After the 2016 ANA report, direct rebates became principal trading. When principal trading gets too much heat, it'll become something else: media investment partnerships, performance commitment pools, whatever sounds most like a product and least like a markup. The holding companies are genuinely good at this part.
The incentives haven't changed in a decade. The money still flows in one direction. The only question is whether enough clients will start reading their contracts before signing them.
Zach El-Amin covers ad tech for The Daily Vibe.
