I've done some digging. From what I've read, searching on Google, I think most of the music videos on iTunes are 480p, it doesn't matter which artist.
For the sake of testing, I actually bought another music video from a different, non-idol artist on Recohoku. It's still 480p...
So for some reason, despite Japanese companies constantly pushing people to buy MVs, they don't seem to have the decency to sell you the HD versions of the music video
Thankfully, UF/H!P are one of the least worst offenders of this stuff, just because they actually upload most of their MVs, but as someone that listens to a lot of Japanese music, this just frustrates me to no end. I honestly don't know how Japanese people put up with it.