Before we start, I am not a lawyer either.
For those of you who hadn't seen it, it was discussed here: https://forums.spongepowered.org/t/question-about-pore/1917 - though bear in mind my responses to that are biased to the earlier days where Pore was simply marketed as MIT licensed.
I would argue that @gratimax and @Aesen are correct here, Pore is doing nothing wrong by saying it's code is MIT - because as it's been said, the MIT licenced code can be sublicensed to be GPL code. They just have to be clear that the distribution with Bukkit, or indeed, linked against Bukkit (the Bukkit licence AFAIK does not have a linking or classpath exception) will automatically turn it into a GPL'd distribution . Otherwise, it's perfectly valid here, it's only linking against GPL and MIT code, the API does not reference Mojang code in any way, and any implementation does not have to either - for example, Glowstone.
As long as Pore is not distributed with anything that requires a Mojang server to run (i.e. the Sponge Forge mod), it should be fine.