I'm not sure that's fair. I suspect Microsoft had to make some concessions to get AT&T to take on WP (they were the first carrier to do so), and with all carriers, give the carriers final authority on software/firmware releases (not unexpected since the carriers are going to be the first-tier support).
I think the DP is Microsoft's way of getting around the carrier sign-off. They've made it available to everyone, waiving all dev fees. Everyone who is aware that a major update/upgrade is coming can get the software now, knowing that support is limited.
If WP had 10% market share and growing, JB might be able to press the carriers about the release. And although part of cause of the low market share may be the carriers themselves, Microsoft is probably doing all they can to get 8.1, with all its improvements and new features, on the streets. Why wouldn't they?