People way smarter than me have theorized about why music has become a free commodity in the post-web era. I don't pretend to have the answers. I think about it a fair amount, though.
Is it because music was easy to digitize and compress into downloadable (and then stream-able) form? Yes, partly. Did the music industry catch on too late? Yes, or so I have read. Are musicians used to giving their work away too readily? At this point, definitely.
One thing that I wonder about is the lack of a tangible object involved in modern music. For example, my wife makes accessories that she sells via the web and at craft fairs. It would be pretty weird for her to just tell everyone who visits her booth, "Hey, just take whatever you want." It would be even weirder if she set prices on her website at $0.00 and then essentially paid to ship products to people.
However, at this point, this is what musicians are often doing. Thank God for live gigs.