It doesn't really mean anything other than be careful who you invite. Our opponents are here, so there's not really much concern about agencies hanging out here - they are. Probably even bake. Though arguing about it is pointless.
Mostly, it's to keep down on brigading. Linking back to the board in some image that gets a bunch of popularity results, potentially, in activists or evangelists for some cause or another to come in and start doing their thing.
Out of sight, out of mind.