Nope, we do have a First Amendment, and it protects bone-headed opinions that are articulated as well as smarter ones. Aside from that, Warren properly informed the Department of State of his trip. He's not saying anything different than many Democrats and 'moderate' Republicans in congress.
FWIW, i think the man is foolish, should NOT be held up as an example of a Christian Pastor, and needs to get a refund for his time in Seminary.
Well, I thought the same thing about Jane Fonda in Hanoi. But what do I know? I thought we were in a war.