I may be in the minority (everybody loves free, right?), but I think this is a bad move. I really don't see it as fundamentally different from Apple deciding that all iPhone apps must be free.
Banning users from charging for their addons is questionable. Banning users from even mentioning in-game that their addon relies on donations is just stupid. If you are familiar at all with WoW addons, you know that the author's site is in the minority of the places people get the addon from. There are a lot of 3rd party collection sites, and there a lot of 3rd party addon installers that install and update the addon for you. Basically, this is like if a different group made Windows Paintbrush and tried soliciting donations on their website. How likely is that that people will go there, see it and donate? Now imagine it was far more useful than paintbrush.
The reason this is colossally stupid is twofold. First, if someone makes a commercial addon, other addon creators will see it and realize it's possible to clone. If it's a really good addon, they will clone it and release it for free. Sounds familiar, no? This is basically a large part of the way OSS works.
The second reason is that addons become work, if the addon is at all complex and popular (aka useful). At some point, you're spending a lot of time supporting the addon that could be spent doing other work for money, playing WoW, or just actually enjoying your life. As codebases age, they definitely fall out of that "enjoying your life" category. This is why donations can actually motivate you to work on an addon when you would have otherwise abandoned it.
The people who take a simplistic view that "other people shouldn't be making money off of Blizzard's hard work!" either do not understand or are too dogmatic to consider the reality. Addons add value to WoW. Blizzard makes money off of addons, be they free or pay, through increased subscriptions. There are numerous users who would stop playing if addons weren't around to make up for the deficiencies in WoW's UI. Addons also very frequently serve as their research department, as you will often see a new version of WoW incorporate the concepts of a popular addon.
This will result in many popular addons being discontinued. It will result in many addon authors losing interest in the game (I used to build addons even once I had lost interest in actually playing.) It will result in many players dropping out of the game because of lack of addon support (WoW updates and UI code changes typically mean that an addon will stop working within a year of being abandoned).
This is financially bad for Blizzard. However, if it's only 0.01% of their income, they will likely not care. I guess the new policy will be a good form of market research to see just how important the addon community is.
BTW, this has already been discussed in much more detail by the people who actually make addons. For those who aren't in the community, I'd recommend you read it to see how it has already killed some popular addons that relied on donations.