Hi alea,
It's certainly possible MS/Nokia is building something like the iTunes model, but there are several reasons to doubt this.
Based on the small market share for WP8 I think their reluctance to release the APIs for TPMs has more to do with waiting for demand to appear before they push the carriers. To do so prematurely could risk the market share growth they've experienced since acquiring Nokia.
Also, if MS/Nokia were to follow the iTunes approach then it seems they would rankle those carriers who are now more wise to being outflanked as AT&T was by Apple. Again, they could put their market growth at risk.
Finally, MS is a close partner of the federal govt, particularly DoD, and I doubt they would want to deal with yet another proprietary Apple-like solution. I believe the DoD plan for 2014 is to complete TPM-enabled smartphone pilots and then expect some level of commercial availability in early 2015. I expect their preference will be for devices that follow an open standards approach.