@g3rv4, I understand it really sucks and feels terrible. But I also understand their reaction.
Don't be surprised about legal writing. He/She as a legal person is responsible for writing in a very clear and explicit way since any misunderstanding might cost the reader or themselves in the future. It just shows their accountability. You don't want a misunderstanding cost you trouble.
I have colleagues working in the legal department. They understand you, but they still have to make sure there is no misunderstanding there.
They are serving enterprise companies, Anything goes wrong can destroy their business. It makes sense for them not to risk anything.
Companies are paying them for the security. Otherwise, there are cheaper alternatives to Slack with somewhat similar features.
Slack could allow custom clients. But I'm pretty sure all major customers will require their employees not to use un-official clients for security reasons.