I saw this as well. The group seems to be pretty damn pissed. My Patriotism says that's some bullshit but the fundamentals of my devotion to America says it's within the rights laid out by the founders (though I'm positive they wouldn't smile at the thought.)
There is no law, but there is Title 4 of the United States Code, often called the flag code. The flag code is violated a lot. Most people are just clueless. I hate to see the flag abused. I even get upset when I see an athlete in an international event with the flag draped over them like a shawl. I've seen patriotic displays placed on a flag which is a violation. Even cities often fly the American flag at half mast for a death. This is often a violation of the flag code. I know most people don't mean to be disrespectful, but I'm just old school when it comes to the flag.