They leak data that enables the marketplace they run and profit from. That's the indirection. They count on this obfuscation (lack of an explicit transaction for the bid request data) to keep people from thinking of them as selling data.

Collectively the participants use this data to optimize their future bids and other surveillance/marketing efforts outside the Google ecosystem.

I'm sure Google would prefer there not be any parties involved that are purely leeches. But they may not be able to or may not care enough to tell the difference between leeches and parties that are still working out their infrastructure and bidding strategy and will begin bidding eventually. Or perhaps making a certain number of low bids you don't intend to win is what keeps you in the game.