The sale of accounts also prevents people from wanting to scam in the first place. If someone wants to scam there is no guarantee it will be successful and if it is not then the account looses essentially all of it's value. On the other hand someone can simply sell their account and receive more of a 'guaranteed' payout.
In addition to scams in trades, these accounts can be used for scam ICOs, coordinated pumps, any amount of psychological manipulation imaginable.
Why the mods here would tolerate such potential misuse of the forum is beyond me, especially when it comes to something as serious as money.
I don't care about the signature campaign aspect so much. If website owners want to pay some unimaginative goons to spam this forum with semi-intelligent blather on their behalf, its their BTC.
I'm more on the lookout for collusion to make certain coins look more popular or trustworthy than they actually are.
Call it a hobby.
I can tell you why. Because there is no way of prevent this situation. They can of course make a rule against it, but then all sellers will just move to other forums or sites to announce their trades. So admins here are just ok with it because that is the easier way.