Oh sure, you can sign your own payloads and install your own keys, but in practical terms this means that people who buy computers will not only have to put up with what the vendor pre-installed - just like the majority of people won't ever "install Linux over Windows", even though lots of people think that this is a good enough workaround - but they will have yet another barrier if they do ever discover that they could run something else.
And I'm sure it's not beyond the skills of the vendors to make installing one's own keys a near impossibility and then claiming it was an accident for as long as it takes before they can then claim that the product is no longer supported.
So in practical terms, it is all about control. We can discuss technical workarounds as much as we like and deny that the technology imposes any particular restrictions, but the combination of one company's continuous strategy of pushing the regulatory envelope and that technology results in a shoring up of that company's position.
Why else are the distributions jumping through hoops? Because they like a challenge? The practical effect of the misuse of such a technology is as much a fact as any aspect of the "it's OK - I can still boot my kernel" technical discussion.