Well that's not strictly true. Presumably there is a contract of some sort, and Slack must abide by it, entitling users to Slack's infrastructure to some degree.

It's not completely arbitrary. Plus there're notions of estoppel potentially at play.

I'd pretty much guarantee their terms of service stipulate they can terminate an account at any time for any reason. It's how most Internet services operate. The only likely addition to that, is a monetary refund if warranted based on the account context.

Pretty sure the contract is going to be in Slack’s favor. They had all the leverage when somebody agreed to it.

https://slack.com/terms