Yes, but these rules usually pertain to stuff other than letters (e.g., you’re not supposed to mail explosives or anthrax). As long as all that is in your envelope is a sheet of paper the post office doesn’t care whether it’s a love letter or neo-nazi invective.
The phone company might be a better analogy. Its business is connecting phone calls between different people, but the phone company doesn’t make it its business to listen in and disconnect your call if you say things you’re not supposed to say. (The phone company may be compelled to let the FBI or NSA listen in to your calls, but that’s a different problem.)
