"Dolby Digital" would be available with a Dolby 2.0 source because it is still a Dolby Digital signal, but it's going to be treated different from a Dolby 5.1 signal because people are likely to want different modes applied to each. As to why it keeps reverting to Pro Logic for Dolby 2.0, my only guess is that it is using whatever the input menu's default mode is set to.

For TV viewing, you may will be happiest letting Pro Logic II help out with Dolby 2.0 sources - many will contain matrix cues that are specifically intended for Pro Logic II to decode. That may be my personal preferences prodding at my keyboard, though, as that is what I prefer to do.