Think about it. Maxed accounts for ladder are expensive, and many don't belong to a single person. Many different players cycle between the account, TOS is breached.

Then 20 win challenges. Most people that are giving their account to someone to achieve the 20 wins personally own their account. They are the only ones regularly playing on that account, it belongs to them. Still, TOS is breached.

However, ladder account trading doesn't benefit the individual in game, though trading for challenge wins could result in abuse of the in-game economy by a surplus of people receiving the 20-win gold prize. Also, the individual gains benefits in game through the trade, unlike ladder account trading.

Not only that, but ladder account trading doesn't involve money between the players of the accounts (from what I know). However, many cases of trading accounts for challenge wins does involve some monetary reward, and not only is account trading against the TOS, but I believe that spending real money to third parties to give yourself an advantage in game is also against the TOS. Kind of like why the 15k tournaments were removed.