Why are you so insulted by it OP? I think that's a little ungracious to assume that because someone made a mistake, even though they were making it thier own, doesn't have the proper respect for the National Anthem.
I flubbed up the National Anthem once.*
I sing it All. the. time. At sporting events. Local patriotic events. Even simple community events. I sing it straight. I don't add all the riffs and extra notes to make it my own ( I don't think it's rude, just not my style.) I know the song backwards and forwards as I have been singing it for years but I still practice it once before I go on, just in case. I have the utmost reverence and respect for it because I come from a very Military family and as such, am extremly patriotic. I also get paid to sing at these things sometimes so I guess that puts me in the "professional" category. I still flubbed it up. The words just fell out of my head and my nerves took over. It happens. I was mortified but I kept going, tried to find a safe phrase to get me back on track, and finished the song... just like Christina did. No one was "appalled and insulted by it", or accused me of not having the proper "respect" for it, at least, not that I know of. In fact, most people were impressed that I didn't just stop and start over or run off the platform crying in humiliation.
Most people have enough sense to realize that sometimes, nerves just take over, even in the most polished performers, and mistakes happen. I would hate to think someone was so insulted by my mistake that they would call me rude.
*by the way, the National Anthem is actually recognized as one of the hardest songs to sing and sing well. It wasn't originally written to be a song. It has octave jumps all over the place, there is no real discernible rhythm or way to keep time singing it, and it doesn't have traditional rhyming like most songs have. And the notes on the high parts are out of most peoples vocal ranges unless they change the key and even still, it jumps so high that it can throw even the most well trained singers for a loop. I am usually impressed when someone gets through it at all!