That's a good question... I honestly don't know!
I'd definitely have less of a problem with it... which is, I realize, pretty dumb as that doesn't alleviate the harm to the artists.
About the only intellectual justification I can come up with is that, as a free service, it's easier to position something like that as a way to discover artists. I think, by selling a subscription, their customers would be more likely to view their consumption as legitimate, and so wouldn't feel compelled to later turn around and buy the music they were listening to.
But again, that's my intellectualizing a position I can't honestly explain.