"Legal and useful" are but two criteria Apple uses to approve or refuse apps. They also care about safety, performance, and design.
IMO its fair for the app store to remove whatever it wants but there needs to be an escape hatch where apps blocked by apple can be manually installed. Since the API apps have access to is secure, this should not be a significant issue.
Apps like Parlor may be full of ToS violating content but users should still be able to sideload the app and the government should be the one to shut them down if the content is illegal.
Coincidence? Probably not.
We can take this all the way to fabricating wires to build computers to make our own servers; at some point we have to discuss the actual issue: where to private and public rights intersect and what is the role of government in resolving the conflict.