It doesn't matter how shitty the major labels treat musicians, or whether they get paid or not. To a musician, signing with a major label means you've "made it". If your shit gets leaked out there onto the internet, major labels will lose interest in you. They won't put up the money to promote you and won't sign you if you are perceived as a risk. There goes your career. There goes your opportunity to make a living off your music.
Every small-time musician I've talked to has expressed favor for stronger copyright controls, both in the form of legislation and technical measures like DRM. I bet the band you see playing gigs in your local dive bar are all SOPA supporters. If it means the difference between where they are now and playing the Hollywood Bowl, they'll support it.