I think most musicians aren't generally savvy enough for the business side of being a working musician. So playing for free and justifying it on some level becomes easier than setting a rate, negotiating, upholding agreements, managing expectations, etc. And to some, as soon as there's a paycheck, it's a job - stress, taxes, a boss or client, failure, success, copyrights, unions, insurance riders - all things that you have to consciously deal with now. If you're really a guy who wholeheartedly believes getting paid for a gig is bad, then I challenge you to hang around more pro (as in profession, paid) players and see if it doesn't become contagious.
To the starving pros, I would say there are ways you can still get work and "out value" the free-birds. You can be a smoking player, but are you a good hang (good attitude) when doing sessions? Do you know how to comp or sight read better than the competitors? Do you play other instruments in a pinch? Do you have several sets of gear for the right job that maybe an unpaid guy might not have? Do you pass gigs along to friends you trust if a gig is out of your wheelhouse or you're booked? Everyone has an edge - find yours and sell it.
People pay for entertainment. Music is entertainment. If you choose not to get paid, that's your right - just know you are entertaining yourself at that point.