Given the post-sales problems with various SoCs and boards, actually delivering a top-to-bottom Free Software distribution should be the minimum just to demonstrate any particular device's credentials in the "openness" department. That's not just an ideological requirement but very much a practical one: no-one wants to have to troubleshoot, say, an overheating system because the software support involves proprietary pixie dust that may or may not be available.
Not only that: if the machine doesn't run a vanilla Linux kernel, there's a terribly good chance that 3 years down the road, you'll still be stuck with the same outdated kernel.
Stefan