The Wii U doesn't support Blu-ray movies and the discs are not officially Blu-ray discs (in the same way that Wii, Xbox, and Xbox 360 discs aren't officially DVDs). The only difference (in all cases) is the logical structure of the discs - physically they're identical. This is enough of a difference to make the disc technically not a Blu-ray. The fact that it's technically not a Blu-ray also means that Nintendo don't have to pay license fees, while at the same time, they get all the advantages of Blu-ray (good capacity, good read speed, established production system, cheap unit cost).
The downside is that Nintendo can't use the Blu-ray logo on any packaging, and can't call their discs Blu-rays (so they haven't, which is what causes topics like this). On the console side, the lack of licensing means that although they have a Blu-ray drive, they can't call it that, can't use the logo, and can't play Blu-ray movies. These are, I think most people would agree, relatively minor disadvantages from Nintendo's perspective.