Personally, I don't see anything majorly wrong with riding on them. As long as no real damage is done, and as long as you don't ride the plaques, there isn't much wrong.
Sure, they are there to commemorate those who have fallen fighting for our country, but it's only riding. I'm certain they'd rather have people making use and having fun on them. You could argue that they died for our freedom, and we're using that freedom to do something we like.
Although, I do cringe when people mess up and bashguard parts. Just be respectful and know where the line is.