OK, have I missed something here? I don't see where he's "advocating violent overthrow of the government." Are you saying that it's against the Constitution to advocate overthrowing the government in a violent manner?
Perhaps you missed the word "buried" in HuntsvilleTxVeteran's post? Since he is so upset about other illegals, I just want to make sure he knew that advocating violent overthrow of the government was illegal too - he / she never answered my question - but yes, to answer your question, it is illegal to do so (see case cite above, as well as "Treason" in the U.S. Const.).