I'm guessing even if I'm on airport property and once confronted, calmly put my things away and drive off, they have no authority to stop me.
Well, actually, the FAA has the authority to stop you, and fine the hell out of you, and it has nothing to do with your camera. You have no business being on airport property unless you're there with the permission of the airport. It's a safety thing. They don't want people wandering around and getting hit by planes, and that would be surprisingly easy to have happen. I have a friend who worked at an international airport and would get me tours of maintenance hangars, etc while crews were moving planes around and tearing them apart to fix them. It's amazing how you can't hear a 757 heading your way until it's almost right on top of you. It's not a place that people should be unless they're being escorted by someone who knows what they're doing.
Back to the photography issue, I take issue with the whole idea that the "terrists" are going to be taking pictures of airplanes. What possible reason would they have to do it? The 9/11 hijackers didn't need to. Who cares what the outside of the plane looks like. They needed to know how to drive it into a building. Taking pics of airplanes landing wouldn't have helped them in the least.
The objection I have to the security theater we've engaged in since 2001 in this country is that it wrongfully persecutes people who are engaged in innocent and lawful activities, while not actually targeting the terrorists. If a terrorist is not going to do something, then why hammer people who are doing it?
If I wanted to hijack an airplane, I would find out how to break through the cockpit door, and how to fly the plane once I'd neutralized the crew. I wouldn't care about how to land it, or where to land it, or what other planes do when they land, or what the tower looks like. Hell, even if I wanted to blow up the tower, I wouldn't need pictures of it. Google Earth would give me all the positional information I'd need to plan the attack.
So what this poster is really doing is saying "Even though there's practically no chance that an actual terrorist will be doing this, if you see a photographer taking pictures of something, you should immediately assume he's a bad guy." Meanwhile the real bad guys are chuckling at all this crap which is directing attention away from what they're really up to. There's no other way to describe it but "wholly stupid."