Twitter allow scam advertisement because I believe they do not moderate scams anymore. Moreover, they do not censor the linking to mixers as well. But If you have noticed the recent development, our beloved BitcoinTalk which is supposed to give full freedom of speech is now censoring mixers. Admin afraid, the forum might get seized for linking to the mixers. What about social media like Twitter and Facebook? Aren't those platforms more centralized and more censored than BitcoinTalk? Sorry for the off-topic mate.

As you know, these platforms do not care about anything except for money. If you pay for the advertisement, no matter what are you advertising. You are allowed to promote anything.