yes but, but...
Having heard a number of cases of manufacturers trying to refuse warrantee or other things if a PC does not have the original version of windows on it that shipped on it, I think that some manufacturers may actually like a bios that stops purchasers doing it.
If you look at HP; their idea of support is telling you to boot from the restore partition. Getting them to even look at a hardware failure will take hours on the phone, with them insisting that you restore the machine to as new S/W and delete every single item off it.
I think that MS should actually ask the manufacturers to include the "any other OS" option. Just think, if you have to hack it to run Linux on it, a lot of people will do it. If the easiest way is to be able to cut MS keys, then maybe that will happen, and then all MS installs will be just as vulnerable as before.