I think technically they can. It’s because they’re downgrading them to a free service. I’m going to venture to say that it’s a money grab on a previously generous gift by slack.
> Slack, a Salesforce-owned workplace messaging app, recently blocked other software firms from searching or storing Slack messages, The Information reported on Tuesday, citing a public disclosure.
On the contrary, not hosting them means they're not losing money either.
It's win-win for them. Either they pay (they won't/can't), negotiate to pay ($$$ at least)- or they move away, all of which increase the amount of available resources for Salesforce.
does sound like it's causing issues that they don't want to deal with if it's not earning them money