I don’t know if this is known (but I assume) as I was trying to get to know the new features of RH7 and using the SubD tools for the first time, I came across the behaviour described in the title.
I did a short video (the clicks are lagging) where you can see what happens if you select your object prior to clicking to extrude SubD. It then duplicates the object as a whole.
If you click on the command first, and then on your model surfaces, then you can select the surfaces you want to edit as expected.
Again, it is just an observation, don’t know if that is intended like that.
EDIT: In addition, looking at other SubD Tools, it would be great to also have the Gumball available when Extruding SubD.