It doesn't matter if it's physical or not, does it? Physical goods for a penny or digital goods for a dollar. It's all irrelevant to the discussion at hand, really. The methods both companies used/are using are what we are talking about here. Columbia House and BMG used low cost to entice people to join a service in hopes they'd stay subbed once they had met their commitment, just as Microsoft is with GP.
It very much was a promotion with the CD clubs, because the low cost of entry only applied once, and not everyone got the same deal -- some people only got 8 CDs, and some just joined to join. Back to the promotion though, after that, you got no more free discs and were expected to pay full pop for all future purchases. Unless of course you cancelled and got another 12 free discs at the promotional rate, which is what people did and likely led to the demise of such clubs, and they'll do it with GP too.