It all depends on how you look at this, I guess that MS have to pay a small fee every time a track is played, add this up for all the free users and it probably amounts to a lot of money, and the only source of income to cover this would be the users who pay for the service, so in effect they are either getting less of a service or it's costing them money.
I do have a music pass, I have about 1400 songs that were ripped from CD's but I now have an additional 2000 songs that I have downloaded over about 10 weeks, I am a bit tight fisted and sceptical of subscription services but this service is well worth it for me.