Put simply: because some on the left hate America:

Last weekend I did something I haven't done in a long time. Something I swore I'd never do again. Something I'm deeply ashamed of.

I stood during the singing of the national anthem. [snip]

I won't stand for the American flag because I won't stand for what is done in its name. I won't stand for the current war in Iraq, I won't stand for the last war in Iraq. I won't stand for all the wars before that. I won't stand for its selectively faulty elecotral [sic] process and I won't stand for its unelected, renegade government. I won't stand for its medieval attitude towards sexuality and privacy, for its violent misinterpretation of Christianity, for its refusal to deal sanely with AIDS and all other global health crises, for its environmentally suicidal stance on climate change, for the hypocrisy of its practices, for the torture of its prisoners, for its executions and its drug wars and its oil wars. I won't stand for any of these things, and I won't stand for the United States of America, or its flag or its anthem, until they change.

Read this closely and you'll see it is more than a garden-variety anti-Bush rant.  George W. Bush isn't responsible for the first Gulf War, nor any of the wars before that. He's not responsible for whatever flaws may exist in our electoral system (they've been around, and been much worse, long before he got here) and Bush has arguably done as much or more to combat AIDS than any president before him.

There are only two conclusions to be drawn from this woman's comments: either she is a partisan hypocrite of the highest order (did she vote for Bill Clinton after he flew home to execute Ricky Ray Rector?  Was she standing for the National Anthem while bombs were dropping on Kosovo or smoke was still rising from the ashes in Waco?) or she truly detests what America stands for regardless of which party is in control. Either way, it's not pretty. I'd be more angry about it if her self-absorbed navel-gazing wasn't so strikingly pathetic.