Simply view it in this manner.
A person who has an account here has some social associations with other people - in other words the other people "know" that person via that account. If ever in future that account has been sold then the socially associated people would not be able to decipher whether they are talking to the same old friend or lurking scammer. This is why account sales leads to scams.
Secondly accounts are sold here to get in signature and bounty campaigns. Which is fairly obvious why they are to be tagged.
@The Pharmacist
Dude I am jealous that morons are PMing you for merits. I wish they would PM me. I would gladly give them something they "deserve"
It wasn't removed, he just isn't in Default Trust anymore. Are people really this clueless?