The discussion is whether they should be able to kick someone off their platform who has been convicted of no crime, because of how integral they have become to the political process.
If it was a power company shutting off electricity to Facebok, Twitter and AWS, because they disagree with their business practices, we would have an outrage right now about how undemocratic it is.
(And I say this with all my distaste for Trumpism)
After all, a free speech advocate and a neo-nazi would both support the publication of Mein Kampf - but the former might well want to be clear they're not the latter. (and of course, to complicate the situation further, the latter may want to disguise themselves as the former)
As to why you see so many posters affirming they don't support Trump - that's because HN readers aren't Trump's target demographics: Trump only got 12% of the vote in San Francisco in 2020. IMHO it's believable that on HN, free speech advocates simply outnumber Trump supporters.
https://techcrunch.com/2019/10/23/just-6-of-u-s-adults-on-tw...
But to give you a partial answer anyway: Many criminals are never convicted. Many _known_ criminals are never convicted, due to lack of undeniable proof. Some criminals are convicted on lesser crimes, just to get them off the streets. There's plenty of evidence to support the position that Trump is a criminal. He may never be convicted but not because he's innocent.