Hosting encrypted zips en masse would also seem to be slightly shady? Sure, there are legitimate uses for that. But if it’s 90% of what your customers share on your platform, you might want to consider that it’s also legal not to host that type of file.

Most of those file sharing platforms get a lot of business from copyright infringers. Keeping files password-protected allows the platform to respond to takedown requests but deal with a minimal number of takedowns. The platform doesn't need to tell its users to do things that way; it's just a natural outcome of incentives. Uploaders don't want their uploads removed. I remember downloading encrypted .rar files to get expensive textbooks even back in the early days of RapidShare. The sites are by their nature pretty sketchy but usually for less heinous reasons than those under discussion here.