Exploiting ingame is also quite a visible offence. If people are doing something on their own server that results in silly amounts of gold/xp with no effort required well that could be punished quite severely when discovered. But on admined servers theres a finite limit to what you can do, and as long as it uses ingame exploits you can combat it. There is no limit to what you can do with executables and dodgy models though, and there is a very deliberate attempt to cheat when using third party stuff. To me there is a grey area with exploits, but its very clear cut when it comes to downloading a piece of software designed to make the game artificially easier for you.
It is also a big **** you to the other players that play fairly, and it sows doubt in people when they see naturally good players who put time into getting better at the game. I don't want it to be something on people's minds when they play, that someone may be cheating because its seen by devs as actually not as bad as leaching.