[quanticle]

I could buy an OEM pc without an OS

I don't know about "without an OS", but Dell is selling a laptop right now that comes with Ubuntu Linux out of the box: http://www.dell.com/us/business/p/xps-13-9360-laptop/pd?3x_n....

And, like others have pointed out, the Surface isn't "locked". Yes, it has SecureBoot and UEFI... just like literally every desktop motherboard shipped within the last 5 years. You can go into UEFI, turn off SecureBoot, wipe Windows and replace it with whatever you want. It may not work very well, but it's totally doable.

If they want to sell Surface because it's an awesome product, why do they need to go out of their way to lock their software on it?

Because the software is part of the product. Apple has shown us that customers do not consider hardware and software to be separate entities. They consider both to be an integrated whole.