Of course it's all about reputation management. These rules are in place to protect the company, not the employee. If an employee does something stratospherically dumb on social media then the company can say "we told them not to" and completely distance themselves from it.

I don't see an issue with a ban. These people's social media accounts are only interesting because of their job, and the status of their job is intrinsically linked to the status of their employer. Nobody would follow these people if they weren't BA cabin crew.