As a musician, I have to point out one thing: the American national anthem is notoriously difficult to sing as written, requiring a grater than normal vocal range. And some with the range need to have it transposed to a lower register (and forget to do so).
So it's no surprise to me that so many try to put their own spin on it...by doing it in your own style, you've won half the battle, at least in terms of difficulty.
That's because the melody is taken from an old drinking song. Of course it's going to be hard to sing sober!
Unfortunately many times it fails, and there are people who think it was done as a deliberate smear of America itself. And they get disproportionately enraged over the performance.