You're assuming that there's a decent amount of people who both have a pressing need for an OS upgrade and wouldn't revolt at the manufacturer charging them to access what is primarily a free update from AOSP. Furthermore, once one person pays $30 for the upgrade, that image can be freely passed around the tinkering communities, so the manufacturer has an incentive for even stronger consumer-hostile DRM.

(Yes, integration takes work, but that's exactly why manufacturers need to push their modified code upstream so it can be tested/changed along with the rest of AOSP. A carrier-blessed release should take QA and some minor bugfixes instead of a horrifically large merge-and-debug.)