What do you mean "take care of them"? If they're not allowed to restrict you from displaying their ads next to ads from other networks, that's the exact thing the spammy websites were doing.
They can simply decide to pull their ads and not do business with websites that spam ads, if they think that's detrimental to them or doesn't provide them any value.
Exclusivity contracts give way too much power to Google at the expense of everyone else, if we could simply leave it to Google to decide which websites they want to place advertisements on.
It's like having a retail electronics store placing bad products from company B next to good products from company A. We don't really let A bully the store into exclusivity either, if A doesn't want to be associated with the store they can simply end their contract.
If they really didn't want to be a monopoly - they'd figure out a way to classify spammers as such and provide a worse selection of ads for them.
Rather, they are specifically locking everyone on their platform.
I do not completely understand your "de facto" vs "de jure" distinction. Unless of course you don't see any other ways of dealing with your clients but a binary yes/no.
Instead, they consciously choose themselves to be a monopoly.