Apple placed a low-ball bid for for AdMob, when they got outbid, they figured they'd simply use their dominant market position instead of out-competing them. This sounds awfully like that other OS vendor that got in trouble for driving a lot of companies out of business by using their market dominance.
As for other advertisers, I'd be worried about Apple cuttng them out of the loop. It's not a matter of if, but when.
So given that Posterous happily lets Google collect stats about all their users and their business, your hypothesis is patently absurd.
Wal-Mart, for instance, may sell newspapers, but they're well within their rights to not sell newspapers with, say, pornographic advertising.
it would be the same if wallmart said, all newspaper advertisements must come through me.