This is a fairly surprising move by Blizzard since the new policy basically screws over some add-on developers who have helped propel WoW to the success it is currently experiencing. Not only has the game benefited greatly from add-ons that addressed deficiencies in the standard UI, but Blizzard themselves have patched in UI updates 'inspired' by successful add-ons, as Lum points out.
This is a clear message that no-one except Blizzard should profit from WoW. They are basically saying, "We'd love you to improve our game for free, but don't expect anything other than warm fuzzies as compensation." It will be interesting to see reaction to this news from developers of popular donation- and fee-based add-ons. How much of an impact will it have on WoW if they cease to develop and support these (or new) add-ons?