I don't understand why people get upset at this type of thing, either, and I served my country and was deployed to Iraq.
Here's the deal: America is supposed to be built on principles on freedom of speech, religion, and expression. If that's really true, then there should be no reason for anyone to get upset when another person, such as this young lady, is exercising her rights to such.
I was a very naive individual when I joined the Army National Guard in September of 2001, just 2 weeks after 9/11. In my previous brainwashed state, I thought that I was doing my duty as an American, and that every able bodied young American should follow suit.
How foolish I was... It wasn't until I was in Iraq for a few weeks that I started to understand that America isn't always in the right, and just because some entitled asshole who calls himself "president" declares it's the right thing to do doesn't make it the right thing to do.
I ended up coming to the conclusion that many other enlightened Americans have. The whole thing was about energy, particularly black gold (oil), and economic stimulus. A few "good old boys" got rich, while a lot of young men and women got killed and physically and mentally maimed. We didn't belong in Iraq, and I have my doubts about whether or not we should have set foot in Afghanistan. I'm not going to go as far to as to say that the WTC bombing was an "inside job," because I don't have any proof of that, and I've learned not to believe anything if I don't have concrete, indisputable proof. However, I wouldn't put it past some of the people who held power at the time.
The point I'm trying to make is that it's a little bit hypocritical for people to blast someone for exercising the rights they hold as an American.
So she took a piss on the flag? What's the big deal?
Now if she pissed in your Cheerios, I could understand you being a little bit upset. Although, after reading some of the comments people made here, I don't think everyone would be upset at that :)