I don't see these two things being anywhere near related.
If you buy an OEM or a retail version of windows, it will install on any machine. There is no locking you to a single machine or anything like that. However Dell, HP and the rest give you either a restore media that is used to push an image back onto your machine, or a special version of the windows install disc that they made themselves with all their drivers, software and setting on it. That has nothing to do with Microsoft any more than Microsoft has anything to do with the settings you put on your own machine. If you disable the wireless on your laptop, is it fair to say that Micorosft prevents you from using wireless internet in Windows? They didn't do that, you did.They gave you working wireless, you took that away, thus it is your fault your wireless doesn't work. Same here, Microsoft gives them OEM copies that should work on any machine, however they customize them to be their own and in the process lock them from being using on machines other their their own type. It could be due to the fact that of course Dell won't include hp drivers or vise versa. So they are keeping the average person from running into driver issues. It could be that they include software that is only licensed for use on their machines so they are preventing it from being used on other machines as best as they can. Or it could just be that they are being assholes. Regardless of what it is, it is not Microsoft that decides what Dell or HP do.
However in the case of Apple they are doing it just to lock you into their hardware.
You are aware that according to the license of OEM windows it is tied to the hardware its installed on and cannot be legally installed on another machine, right?
The only real difference between the 2 is one locks you to a single machine legally and the other does it physically.