I don't know anything about this person other than what's on his Wikipedia. And what that says is: he's a 65-year old former diplomat and noted human rights activist with (as far as I can tell) zero history of violence or terrorist acts.

If the government seriously thinks he's capable of following through on violent acts because he wrote a Tweet saying "I support Hamas", I suspect the government is badly mismanaging its anti-terrorism resources. If the government doesn't think he's going to follow up that Tweet with action and is "merely doing it because the law says they can", that's pretty much the definition of harassment.