It is worth noting music isn't alone as many parents and kids fight quite a lot over homework too. I've heard from many parents they fought quite a lot about homework, but very little over actually homeschooling. I suspect the difference is that the kids are more tired in the evening when they are doing homework and also parents have control to modify assignments that don't work for the kid for whatever reason.
I'd say there are lots of reasons why music is the battleground for many though.
1. It is expensive so parents feel stressed if kids don't take it seriously.
2. Many parents have unrealistic expectations expecting music to give their kids discipline, work ethic, etc. I've often heard it suggested on gifted boards for what essentially amounts to a substitute for a good academic fit.
