Well using your previous example of Intel giving Dell volume discounts versus a small vendor, yes that in itself is legal. But don't forget AMD successfully sued Intel under antitrust laws for giving discounts to major vendors like Dell in exchange for preferential use of Intel chips over other competing chip makers.
https://en.wikipedia.org/wiki/Advanced_Micro_Devices,_Inc._v....
This deal between Google and Facebook is strikingly similar. Google is giving preferential treatment to Facebook that it doesn't currently offer to other ad buyers and access to analytical data it also doesn't provide to other ad buyers in exchange for Facebook committing to spend a certain amount of money on Google ads. But more damning, according the article, "the two companies had predetermined that Facebook would win a fixed percentage of auctions that it bid on." This clearly gives Facebook an unfair competitive edge over other advertisers and amounts to quid pro quo, which is illegal, just like the Intel antitrust suit.
I'm not clear what part of this causes that. The whole point of this change is to up the price paid to publishers got AND offer more options to ad buyers. That seems like an improvement for everyone.
Google, Facebook, eBay and other big tech companies are very good at walking this line: we control the market but we don't abuse that control.
So either someone has to show abuse or we need to change the law...