if it is more socially acceptable to be rude in songs. Song lyrics certainly have a long history of being bawdy, satirical, insulting etc.

a song might gain some implied distance from the performer by frequently being someone else's creation, even though that's not always true.

a song / minstrel might also fall under the general dispensation for rudeness or critique which was normally granted to fools / court jesters in the past.

I think a song gets away with it because people often pay less attention to the words. Carefully crafted rudeness in song or poetry is less direct than speech, too.