Rock Grotto and iPodStudio set the precedent. After those two, it was easier to just make a rule -- no promoting of forums on head-fi that are in competition with head-fi, directly or indirectly -- and follow it with everyone. He actually does get along with a couple people here -- grawk, in particular.
It actually makes sense, from a business perspective, but certainly seems fascist the first time you're hit with it. I mean, if you were an auto insurance company, would you provide links to other auto insurance companies? Alright, yes, you would, if you were going to take on the big names, and claim that you were better than them. I mean, how much sense would it make to say, "we can beat the insurance company that cannot be named that starts with a 'G' and ends with 'EICO' by 15 cents or more"? No, if you really could beat their prices, you'd want to be able to say that you could beat their prices, and exactly whose prices it is you were beating.