Even without Microsoft's interference, it would still complicate logistics. Two OS options means double the number of SKUs. Some manufacturers offer this option but I wouldn't hold my breath that retailers want to bother.
That sounds like excuses. They could even make it pre-installed but require to enter a code on first boot to unlock the encrypted HDD if they really wanted a single hardware SKU but I doubt that it would be a real problem in the first place.
Yes, Amazon and other retailers will have one laptop with OS, one laptop without. They need to track two different SKUs even if the conversion is easy. When upgradeable, SSD or RAM are also easily "convertible" but the SKU still needs to uniquely identify that "thing" that you have in stock or in the manufacturing pipeline.
