I think part of the difference is that Francis Scott Key didn't claim ownership of the melody. I don't think his brother-in-law did either, who I've heard was the person who actually put the words and the music together.

But I agree that fitting new words to existing melodies is a time-honored tradition. It's also a perfectly normal thing to do. I've done it myself. Grabbing someone else's national anthem might be a bit over the top, though. And we pay for stealing the Star Spangled Banner at every sporting event in the country. Do you realize how many people who can't sing that song do so anyway? And how bad it sounds with the current pop music pseudo-soul vocal stylings?