Been to a couple rallies in my day. Not having seen the sign, there are some security issues.
For example, if it had a stick in it; it can be considered a ‘weapon’. If it is above a certain size, it can be used to conceal a weapon.
Wasn’t there, didn’t see what happened - but I do know that each rally has certain ‘rules’ in place for the safety of all. These were the rules my wife and I had to follow when we went to Crawford, TX.
Sporting event don't allow wooden pools on flags, but you can use PVC.
It can be just as much a weapon as a pole.
Size of signs, language, etc., is pretty much covered in a controlled rally just like sporting events.
A coordinator, security or anyone associated with the event can have the sign, poster, etc., removed if rules are not followed.
In the cafe, the owner probably didn't' mind rally signs/posters.
Security of Newt probably removed it because it was anti Newt.
It happens.
And as you wrote, I too was not there.
Been a coordinator for a gubernatorial race and one (of many) at an RNC. Seen it all.