As mentioned on a comment a little below, this was designed with security in mind. Requesting mouselock can only be done together with requesting fullscreen mod

As mentioned on a comment a little below, this was designed with security in mind. Requesting mouselock can only be done together with requesting fullscreen mode, which is only possible in response to user input (like pressing a button) and after that requires explicit authorization. Afterwards, pressing escape will always leave both fullscreen and mouselock.