I think the question ought to be 'why do people want to learn racist songs?' - after all, they cannot sing a song unless they have made some sort of effort to learn it. Even reading off a page means they have made the effort to find the song lyric and its tune...

It also depends a lot upon whether the racism or racial stereotype is what holds the story together, or if it is merely a throwaway phrase.

For example, the line 'rich as any Jew' (A Mon Like Thee) can easily be changed to something that sounds like it such as 'rich as one of you', which doesn't change the sense of the song at all.

What offends some doesn't offend others...