As a musician (I've done it as a job although nowadays I do it for fun) I find the idea of unpaid work of that kind horrific. I've gladly donated my services as a musician for events for charities, community groups and gigs with great exposure and have always been happy to go to jams (where usually only the house band is paid).
I'm all for personal choice, if any backing musician was offered union rate but said "nah I'll do it for free" then good for them!
But professional entertainment isn't free. Professional accountants aren't free, professional footballers aren't free, professional musicians aren't free. Any one of those groups may include an individual who may volunteer their personal services for free, but no group is free by default.
It seems somewhat shameful to me that Palmer essentially had to be bullied into paying her backing musicians. She's no better than those pub landlord types who expect a professional band to play two hour long sets for a beer each, nevermind the fact that a few beers doesn't even match the cost in petrol to move a band and their gear to the venue.