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.