In the case of the supermarket, the person who is banned could pay somebody else to buy stuff for them. In an extreme case, that somebody would have to be the government preventing the person from starving.

In general I think private businesses should be allowed to decide who they want to do business with.

In the case of Twitch, the question to me is what is the contract the users enter with Twitch - what promise does Twitch make? If it doesn't promise anything, users invest at their own risk.

I had no problem with hosts kicking off Parler, and I don't really have a problem with this Twitch policy, but those are very different things than the grocery example. Social media companies, webhosts, etc. are directly enabling speech as a core part of their product. As was mentioned above it would be against the principle of not coercing speech to force one of these companies to continue serving a customer.

But I really do not think allowing someone to buy lettuce should be considered a form of speech, or even an "association". Banning someone from using a grocery store - or perhaps more relevant to this example Amazon.com - for actions completely unrelated to the service is wrong IMO, even if it were somehow restricted to 100% accuracy rate of only blocking very bad people.

I haven't seen it happen yet, but if it becomes in vogue to worry that selling a random material good to a customer is some kind of endorsement of that customer, that will lead to a very concerning situation down the line. It is now all too easy to gather information on people, potentially dating back decades. I don't see any reason grocery stores should know anything about their customers outside of what can be observed when shopping.