They've got the permissions set up on the work chat server to allow anyone to create chats at will. Those chats can be public (listed in the search and anyone can join with a click) or private (can't be joined without an invite from the owner of the channel).

And they don't discourage non-work-related chats for... team building reasons, probably? There's one for "Video Games" for instance.

I know the guy who made the "coronavirus" chat and he 100% did not intend it for conspiracy theories. The whole IT department was in the process of going remote when that chat was created and that chat was intended for everything from helpful tips for working remotely to news/rumors about when/if we might be going back to the office to news about death rate statistics and such.

And this conspiracy guy had (still has, actually) a deep-seated need to proselatize for the conspiracy of the week 24/7/365. So he just decided that was as good a pulpit as any.

Shortly after the shitstorm started, three levels of management above both me and Conspiracy McGee entered the chat. They didn't end up doing anything. (It fizzled before they had to take action.) But I'm sure they all had their fingers hovering about 2mm above the "shut that shit down" button.

Now, all that said, there is a chat on the work chat server dedicated to the conspiracy podcast "No Agenda." And I'm pretty certain it was created by Conspiracy McGee. And I'm pretty sure my/his direct boss is in the No Agenda chat.

So, I guess the short answer to your question is that they don't want to shut down non-work-related chats so as to pay some lip service to team building and not appear too draconian (while at the same tacitly encouraging a culture in which it's not really acceptable to spend too much time in those chats rather than furiously typing code). And the company's management is sufficiently right-wing as to not get that allowing conspiracy theorists to conspiracy theorist is eventually going to backfire on them, so they don't see it as dangerous. So they see it not much unlike having a chat about the latest Mario Kart game or the Marvel Cinematic Universe or whatever.

Hopefully that answers somewhat.