You can choose not to delegate it, but then how long and who should maintain the list of names that should not be delegated? Who should pay for the work to implement it, and who should pay for the maintenance for said system, and for how long? Should there be an appeal process or some kind of oversight, and who should body those roles?

In theory the law makers could specify this in law as form of registry regulation, but no one want to do that just to address one or two court cases where this question comes up. The registrar of last resort doesn't give any direct answer to those questions either.