[jeffdavis]

I don't approve of scaring people like that of course.

But it's possible that they thought you were carrying the protest on into the train station or some other place which they had safety concerns about.

[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.