Digi is actually a relatively small and lean company they don't always have the resources to teat every version of a software or every hardware platform. So they have to make some choices.
If the Buisness Software Alliance or one of Microsofts inspection teams shows up at your buisness from what I understand you better be using XP Pro or Vista Buisness in a commercial environment be it manufactoring, a partition farm or a recording studio. So approving Buisness is a must for Digi.
After that you look at the three Home versions and try to maximise your investment of time. I would think Ultimate would present the most challenges and is also more likely to be shipping on the type of hardware PT likes to run on. It also has all of the components of Home Premium and Basic.
I don't speak for Digi but I'm sure that their thinking must have been somewhat along the same lines.
Truthfully why not wait a couple of weeks and see how its running on the various versions?