But it's sensible to ask why it's being sung.

Absolutely. Motive is everything. I sing Gus Elen's "The Postman's Holiday" - a great song but with the opening line, "I works just like a nigger and I isn't very strong". In a club of like-minded people I suppose it would be OK - with an appropriate introduction - to sing it as is. But I don't - because just such an introduction would draw even more attention to the line.

So I sing, "I works just like a navvy..." - same effect and no-one's offended. The song works just as well, it won't wither and die in any way because of my editing, and the original words are preserved in sheet music and on record for those who want a snapshot of a typical commonplace view in Gus Elen's day.