Unless you've more money than sense, you don't buy a CD (especially at inflated GB prices) on spec. So you listen (for free) before you buy. makes perfect sense.
For the grown ups whose social life is restricted compared to a teenager's, there is less chsnce of hearing new music in a social setting, only on radio/tv or the net.
These same media companies are not happy to have their output advertised for free on the net, and yet spend tens of thousands to have their output played on radio and tv programmes, and hope we listen to them and buy the music!
Not a new phenomenon anyway, remember Radio Luxemburg and the twin cassette recorder!
But I'd sooner hear live music anyway. (well, sometimes)