Because the claim is that they're monopolizing data they own contractually, when they're offering it as a service to other businesses too. Say what you want about Twitter's death spiral, but their $100,000/month API is an equal-opportunity offering.

Is it shitty, exploitative, barrel-scraping nonsense that has no place in the industry? Sure, another bump on the proverbial log. Is it monopolistic? No, it's cynically hegemonic.