Regarding the First Amendment issue, courts in several states have cited the First Amendment to throw out tickets some cops have purported to issue to people who flashed their high-beams to warn of an upcoming speedtrap. In that sort of situation, if the state has a statute like the one kphoger quoted, the cop could probably issue a ticket for violation of that statute because it's a content-neutral restriction—as kphoger notes, the statute doesn't make a distinction as to why you're flashing your high-beams. But if the cop issues a ticket strictly because a driver was warning of a speedtrap, the driver is likely to win.
With all that said, some people have apparently decided that the longstanding custom of flashing your brights a few times at someone who is blocking the left lane is an asshole move and I'm not sure why that practice is suddenly being deemed taboo all of a sudden. It's long been universally understood as a simple request to get out of the way. However, here in the DC area there's an annoying fad where people who don't like something you do will turn on their high-beams and keep them on as though it's some way to "teach you a lesson" or "send a message." That's stupid, rude, and dangerous, especially if you have newer headlights.
(I didn't watch the OP's video, BTW, and at first I didn't realize it was a Crash_It thread. Referring strictly to the subject line, however, I do think left-lane camping is unfortunately all too common and there's no reason for it. It's one thing in urban areas with heavy traffic—often it's simply impractical to get over. It's quite another on rural Interstates where there's simply no reason to be in the left lane. In Maryland, I sometimes feel like I'm on a British motorway—Maryland drivers are so terrified of the right lane that you can usually get through faster if you keep right.)