[Broken_Hippo]

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.

[jeffdavis]

My guess is they are worried about protesters collecting in a train station, which could be a serious safety concern.

They see someone with a sign, and they try to get him to move along quickly before (potentially) a train load of additional protesters arrive.

I don't really know all the facts here, I am just offering an alternative way to interpet the facts provided.