Music labels are extremely protective of their products these days.  If you make a melody too close to something they own or even have music faintly playing in the background in one of your youtube videos, you're subject to legal action.

Apparently things weren't always so strict.  When I heard that the melody of our National Anthem was "borrowed" from an old drinking song titled "To Anacreon in Heaven", I was expecting to hear some vague similarities between the two...that was not the case.  They just simply changed the lyrics.  The video shows a recording of the original version.