I assume that all the music is owned by Victor, so I figured the same thing. They needed some music for a certain feel, and the MacII stuff fit the bill.
What I think is interesting is that II, 7, and Plus music gets recycled all over the place (especially on the drama albums), but original SDFM music doesn't...except on the Macross Classic audio drama.
There are a few places in 7 where playing DYRL would be appropriate, but they never do...unless is sung by Mylene...I wonder why...