[Molly]

I'd say that the extent to something is done is rather relevant here. It's when things are taken to a large scale that they become a real issue. And as soon as things happen on a large scale, they also tend to become public knowledge, like the EULA transactions on WOW that you mentioned. (On a side note, that's a bad example in this context, since no Mud Administrator can really be held responsible for what crooked players are doing, especially outside the game.
As has been pointed out before, the policy can only involve Admin-to-player transactions, since they basically have no control over player-to-player transactions.)

Now, if the official policy is 'No in-game-rewards' but some shifty Mudowner really wanted to make a considerable income from handing out unofficial ingame rewards for money in spite of this, it would make little sense to be totally 'opaque' about it.
After all, if the shifty Mudowner wanted his shifty players to actually pay him some money for those shifty rewards, he'd have to make the players aware of the possibility in some way.
Otherwise he'd just get the few-to-none legit donations that he'd be getting anyhow.

So, how do you think he'd go about it? By approaching each of his players privately over tell and inform them about it? Not very likely. By word of mouth? That would spread the bad rumor very quickly.

Added in retrospect:

Also, since most players and Mudowners are not crooked, and since pay-for-perks is a not only a legitimate, but also obviously popular system, wouldn't it be a lot more realistic for a player who wanted that kind of game to choose one of the MUDS that checked the b box, rather than going on the prowl for a potentially crooked Mudowner who checked the wrong box on purpose?