A sign for a proscribed terrorist organisation, which its members are backed by a Russian plant (Fergie Chambers - now hiding in North Africa) and have damaged military aircraft, military facilities and attacked police officers with sledgehammer. Things that, in the US, you would have been shot for.
The protesters could have waved any other sign around in support for the cause without any problems, as hundreds more were doing, but not that specific organisation.
However the 500 people are at best naive victims of their own incompetence and being used as political pawns, not for the cause but instigators.
All this stuff is rather less black and white than it sounds.