Can't access your mail archive going back a decade - could be a huge problem for some people
Sure, it's not as bad as getting sent to the gulag. But the problem with most of these tech companies is far bigger than not being able to post a meme.
I do believe you're misapplying Blackstone's Formulation. I think it's perfectly fine to presume guilt in some cases when operating a public service. This can be reformulated as making users go through extra steps designed to screen bad actors. We in fact do this at places like airports (and increasingly schools) all the time.
For instance if your user happens to be an unwitting botnet member (or even if he has a newly assigned IP previously belonging to one) - his IP will be suspicious and you can "punish" him accordingly. Also Blackstone's ratio was 1/10, not 1/1000.
Got sent to the gulag - well now you have a major problem. Can't post meme on reddit - oh no!
Websites generally aren't obligated to accept all users, precisely because we as a society decided that it's not really a big deal if you can't.