Because they paid for the right, just like Viz did when they released the the manga for DBZ's content. I don't blame them for associating it with the first highly successful DBZ anime.
Okay, in honesty I did overlook that the whole thing was simply "Dragon Ball" in Japan. My bad. Still, the key point is that the content was adapted as Dragon Ball Z for animation. Moreover, everything important that we think of when we think of DBZ is basically in Kai.
Besides, DBZ is a behemoth mostly because of circumstance. They stretched it out to keep pace with Toriyama and eventually adapt all of his work, it wasn't because they were using artistic license to add to the show by having the Frieza fight last for an eon or two. If the manga had been completed or almost finished before the series started, we would have gotten a show much closer to what they attempted with Kai.
The issue is too small to really harp on for very long, so I'll end there. Suffice to say that I think it makes sense and that it doesn't even amount to a sin even if it's unnecessary. Compared to the general good news, it's definitely nothing. I'm much more interested in the probability of having an adaptation of Namek that doesn't include corny jokes that were written in 1999.
Yeah, I noticed that tidbit. Good job finding that out, I was wondering about that myself. I think you're right, the HD masters are the most likely. Hopefully Toei won't get stingy like with the Dragon Box video, though it's comforting to know that the 16:9 version is competantly done if that's what it comes to.