Plagiarism

This talk about plaigirism was  very interesting to me as a musician, mainly because of the differences between the way it is approached in music and in writing. so many things seemed to be plagiarism in writing that aren't in music. for example, If I want to write a set of variations on a theme by Bach, I don't have to pay a fee or even give much credit, I can just title the piece 'Variations on a Bach Theme' and it would be more than acceptable, IN FACT many amazing pieces of music are writte in just this way, the Finale of Brahms 4th symphony is based entirely on a chorale by Bach. however if I did something like this in writing, there would be extreme and dire consequences, not to mention the moral implications and punishments involved with such a crime. Overall, I understand why plagiarism is wrong in writing, and have never done it before and don't intend on starting now, but the thing that troubled me is why it should be so different in the musical word, after all writing is an art as much as music is.