* all these websites account for at most 1% of Google Ads network reach by the number of page views

* since these websites are to be opt-in, the advertisers' competition will be much lower, probably 1/10 of the usual click price

* since the number of advertisers will be low, and only marginal advertisers would opt-in the marginal website list, the relevance of ads (and thus the probability of a click) will be also much much lower, probably 1/10 of the usual click rate

So by banning all marginal websites Google loses only 0.01% of revenue from ads network. If my back of the envelope calculations is at least order of magnitude correct, this does look like a very little price to pay for relief from a headache associated with working with questionable websites.