Carrying the sign is a danger now? How about printing it on your clothing? Must one cover up to go outside and only protest when one is licensed to do it? Could he have worn a full-body protest costume instead and traveled in it? If not, how is this more dangerous than a Big Bird costume?

There are multiple ways to handle a safety concern, one of which is just by talking to the individual and observing. Someone with a sign acting like all the other passengers isn't much of a worry, I think.