I disagree. Lots of the streams I enjoy are largely based on the personality of the streamer and their interaction with viewers. If you start removing their freedoms of what they can and can't say that would heavily impact their interactions and limit their opinions/speech. Yes it's true that most of the dialogue isn't heavily political, but politics is a wide sweeping topic. What if they are bringing up legitimate problems with the game, or the parent game company? Totally reasonable and interesting opinions that actually happen all the time on streams would be in danger of getting banned. Not to mention discussion of everyday life outside of the game which is also commonplace on many streams.
Just watching gameplay with no opinions and interaction is a watered down boring stream in my opinion. Maybe there are times when that is what someone wants when specifically looking for gameplay tips and how pros play, but that is only a subset of game streaming even by professional gamers. If the streamer is a sponsored pro, sure they have to worry about those sponsors but what kind of advice is it to tell them they should have no opinions? It feels short-sighted of the sponsors because they are limiting the potential of the stream and if they already sponsor someone, why wouldn't they want their opinions heard?