I feel like this is hard to argue in general. Twitch really isn't all that different than live streams on TikTok, YouTube, or Facebook which are general communication platforms where creators can get partner status and share ad revenue.
The thing that ties Twitter, Twitch, YouTube, Cloudflare, Wordpress, Facebook all together is that they're broadcasting corporations available to the public at large. The value prop to their users is the megaphone. How it's accomplished from a tech perspective shouldn't really matter.
It's the legal issue, though. I personally think that laws against gambling are none of the government's damn business, and if Twitch is doing this only because of laws, then I don't like it, but blame the government, not Twitch. If gambling were legal, and Twitch decided to ban it anyway, I would hold it as equivalent to them banning porn on their platform, and totally legit, at least as long as Twitch was not a monopoly.
I don't think this is comparable to other sites like Twitter banning users because Twitch isn't a communication platform, it's an entertainment one and only allowing certain kinds of entertainment is par for the course.