Nah, it's a contractual obligation that you sign when you purchase the property or move in.
There are certain things that they do that are unconstitutional, but agreeing to keep your house in the same aesthetic theme as the rest of the 'hood isn't really a restriction on free speech or anything like that. There are, however, some things that are superseded by federal and state law that you may be able to get away with. Flags, radio antennae, medical devices (yes, doctors can write prescriptions for lap pools - boo ya!) or handicapped access devices like ramps, elevators, etc. are protected by federal law and nobody would dare try and slap you down for it without getting their asses handed to them in a court of law.
However, since there's no amendment to the bill of rights that ensures that you'll be able to grow your own food on your own land, gardens and things can be restricted.
What a pile of bull.
HOA's only serve to piss people off, since most of the things they restrict as eyesores tend to be already covered by local zoning laws. There's an HOA that spells out that you can't have an eyesore vehicle in your front yard without it being covered. That's identical langauge to the local zoning regulation regarding inoperable vehicles.
WHY?!
Goes back to my argument about gun control, to an extent. If it's already illegal to kill people, then why make the devices used to accomplish the task illegal?