If you read clients terms of use they don't allow illigal content to posted.
Discussion
In practice, clients cannot prevent users from posting illegal content. Any censorship must be initiated by the server, the webhost, or the state.
Plus, part of the reason I'm using Amethyst is because it DOESN'T have any content requirements in its terms of service. Not in its APK/Github/F-Droid version at least.