Some people in the MSM are saying that the flag on sarah palin bus is illegal?

as many people know palin is touring the county on a huge bus . and on that bus there is a huge american flag painted on it . well according to some ppl in the media this is illegal . i never herd of anything like this before . can anyone tell me if this really is illegal . and if so then why is it legal to burn a American flag . but you can not display an american flag on a bus .