"I found this token from clicking an ad", said nobody ever
I don't think this type of news should affect the prices at all but it always seem to drop the market. Ads for ICOs was a bad idea in the first place and most likely scammy. In the long run this should be a good step, most good tokens grow organically through word of mouth, not pushing clickbate advertisements.
Except that most of the time, when these companies announce these bans, they way too vague in terms of scope and enforcement. I think most people can agree that ads for ICO are scammy - but what about to promote a blockchain project? Ads for events related to crypto, even educational ones? Do they count as "Currency exchange, trading, and related services"? There is a huge difference between safeguarding users against scams and suppressing the cryptocurrency ecosystem in general, and its super difficult for outsiders to predict what the ultimate motivation is. Even harder to predict unintended consequences. Twitter, at least, is also banning ads for exchanges and wallets except for public companies listed on major stock markets - which raises it's own set of issues.