Per your sig I understand you're probably better informed than I am on these matters.
I do wonder though whether or not you are correct in this case.
Let's say I've got a piece of software and I decide to publish that under the GPL.
Somebody sends me a piece of code to include in the software.
I add it, and release it, still under the GPL. Then later on, I decide to release the code under, say, the BSD license - including the bit that was contributed by Somebody.
Whether or not I can do so depends on what license that Somebody gave me when they handed me the code. While it might seem obvious that they contributed to something that's under the GPL license and should therefore also be GPL, what they actually did was contribute to a codebase - a codebase under my control, and one that I can slap any which license on that I like.
At least, that's my understanding. So my questions would be: 1. what license did authors actually give Wikia for their contributed content (explicitly or implicitly), 2. does the license of the greater work at that time trump the license on the contribution and 3. why?