It's because you can only have a finite amount of inbound delegations (for performance reasons). So if there was no whitelist someone could send 1 rc to everyone and completely block you from getting any other delegation.

There was other ideas like being able to "undelegate" as a user but ultimately that was the decision that was done in the original design so I didn't change it since they probably had good reasons. maybe we'll revisit it in a future hard fork if slots prove to be too cumbersome.