The BIOS vendors will surely support linux as a checklist item. But they don't sell to consumers. They sell to PC manufacturers. Sony and Dell and HP's laptop divisions don't give a hoot about linux. If they get a directive from MS to turn on UEFI secure boot with MS's key, that's exactly what they'll do. What they're not going to do is write a user-supplied key UI in the BIOS allowing the user to install their own keys. Even if the BIOS vendor provides one, they won't test it. Or might even disable it to "reduce support costs".