There's a post on them somewhere, but if I recall correctly it's not that they aren't allowed, it's that there are certain restrictions on them.
Which I think pretty much boil down to "don't abuse it".
So for instance if you have a single charity mule and you collect extras to give away, that's a different matter than having twelve accounts and collecting with all of them and then selling the extras.
But where in the spectrum the specifics falls, that I don't remember.
So, okay, let's look it up.... under "Help" is the "Rules" section, and here's what it says about mules:
Mules:
Mules (additional accounts) are permitted in moderation. This means you may own additional accounts for things like for example item storage or shop managing purposes.
As a guideline: 1-2 Mule accounts are fine, everything beyond that is critical. If in doubt, contact an administrator and ask about the legitimacy of additional accounts.
You may not create accounts with the sole purpose of gaining additional items for your main account, additional votes in KOfK features like polls or contest votings, or any other aspect of the site where having addition accounts will give you any kind of advantage over users with only one account.
Should we determine that your mule accounts are breaking these rules or you created too many accounts, these accounts will get banned or deleted without prior warning and your main account might be hold accountable with warnings or bannings depending on the severity of the case.
All accounts are bound to the rules of The Kingdom Of Knuffel.