If you are buying a new computer without an opertaing system and want to use XP Home, thn yes, you need to buy a disk.
System builders are allowed to sell computers without an OS so you can choose what to install. You may want to run Linux but the m,anufacturer doesn't do Linux installs for example.
It's like saying car manufacturers shouldn't be allowed to sell cars that can break the speed limit.
-------------------- OK OK I can't go any faster! aaaaaaaaaaaagh!