That just isn't true, especially on x86. I can and have loaded my own keys and made secure boot validate against my keys to force it to only run what I wanted it to. The standard actually mandates this so it's pretty disingenuous to state that secure boot on a computer prevents the user from running whatever software they want to.

The user is always free to control secureboot except for on windows phones, you might have a point about it being a bad thing for mobile.