I understand the difference between the Pledge of Allegiance and the National Anthem. But, did you notice, that out of respect, everyone else there put their hand over their heart? I always do, and, although I am sure some do not do that, I have never seen someone who is running for President, not do it.
Maybe I am overreacting, but, I just think it is done out of respect, like everyone else on the platform did. I see it as a sign of either stubborness or rebellion since everyone else there put their hand over their heart.