I ran into this also. Here’s what I think the best behavior would be-
similar to formatting cells in MS Excel. If all of the selected posts have category A, then category A shows up as checked in the bulk edit categories window. If none have category A, that checkbox is blank. If some do, it is grayed out. You can click it to make all selected posts have that category or not.
This of course applies to multiple categories.
I think that’s the best of both worlds.
