I'm not sure why this rule exists, but my guess is that it's there to prevent dramas over losing names. Either way, it's been there for a long time and I have always followed it - if I didn't like the name of an alt and it was far from level 31, I just deleted it and made another one. So I don't know why other people can't, and I see lots of people who try to cheat this system.
As long as I can, I will keep denying your efforts to evade this system, until hopefully at least one understands and stops taking us admins for fools.