I thought the whole Curse and WoW Interface vs WoWMatrix scuffle was over, but the people at WoWMatrix have just fired back an "FAQ" giving their side of the story. I put FAQ in quotes because I'm not sure these are frequently asked questions so much as questions Matrix frequently wants to answer, but that's OK. I like the format.
Here is the situation as they put it: The Matrix people are respecting Curse/WI's demands that they quit using Curse/WI bandwidth, so as of now you can only get addons through WM that are hosted on the WM servers. They are hosting addons on their servers that the authors asked to be hosted there, or that are distributed under a free license (such as the GPL) that allows such re-hosting.