I'm pretty sure that it's in the TOS.
Back when a lot of people were leveling toons in World of Warcraft and selling those accounts then for a few thousand dollars on ebay it raised a big stink if they got caught because blizzard would hit them with the ban hammer. The reason being is that even though WoW players pay for the use of an account, the account and anything related to the WoW game belongs soley to blizzard entertainment. It is stated in the TOS so anyone then laying claim of ownership was actually guilty of theft.
I'm near certain that TS has a similar clause but I can't be bothered to look for it at the moment. Ultimately its intended to protect the rights of the franchise.
