There is a loophole in this. I did a search of these forums to see if there was anyone that posted any information about it, but couldn't find it. I don't know if this loophole is an exploit, but the Paid Character Transfer service allows a Death Knight to be moved to another account which already has a Death Knight on the realm the character is being moved to. So although it says you can only have one death knight per realm at any one time, the Paid Character Transfer service doesn't check to see if the destination account and realm already has a Death Knight. Maybe this will change in the future, but as of today you can transfer a death knight to another account and realm which already has one. In addition, you can create another death knight again on the source realm because it won't have a Death Knight as long as you still have another 55+ character there. So someone could create other accounts by buying the game then with the account with one level 55+ character, move the death knights over to the other accounts. Plus if the other accounts were created with Recruit-a-friend, the new death knights can work together to level 60 and get triple experience.
The transfer of a DK after it's creation to another server using paid character transfer is working as intended. It's not a loophole.