They would either have to make vendors to check eligibility or force system builders to register to MS and then MS would give them a system builder code which must be used to buy OEM Windows and to activate it.
Seems like MS doesn't really care about it as they haven't done anything to it. People will always use the loopholes if possible
The only loophole here is the fact that the installer does not enforce the license requirement to use the OPK, which would also require the OEM/System Builder to register and identify themselves to do the required pre-install. At least in the US we have this thing called the first-sale doctrine, which is what enables one System Builder to transfer an unused license to another System Builder and lets NewEgg and others distribute copies for Microsoft. (This right is also explicitly defined in the System Builder License).
Similarly, the installer doesn't enforce any eligibility verification for retail upgrade licenses, and they can easily be installed on "bare metal", so why not recommend folks buy retail upgrade editions (which are intended for consumers) instead of OEM (which is not). They cost about the same, but the upgrade has fewer actual restrictions on actual use.
Either way you are using a legally obtained product outside of its license...
B