Wowhead is permitted, they have an agreement with Blizzard if i remember correctly, so I think this might be part of why they can do it.

But I know I have seen wowhead listed on the community site at one point or another.

I think charging for an add-on SHOULD be allowed, personally, someone took the time to write a program, why should they be denied monetary gain for their work.

Mat, you don't write articles for WoW insider for free! So why should someone else, who put time and effort into something be denied. However, I must add that the terms and conditions say NO, so they should abide by the rules of the game.

@arrae, the difference is that even though Mat doesn't write the articles for free, the articles are still freely available. WoW Insider lives on ad revenue (afaik anyway).

I could pay someone to code a WoW addon for me that does what I want (because at that point I am paying for his work from scratch, not an addon), but the author could not then turn around and charge others after he uploads it.

@Snuzzle
Not really different. One of the things Blizzard has prohibited addon writers from doing is adding any kind of advertisement in the addon, or even asking for donations inside the addon itself.