I'm just a Canvas Admin for our institution, but I can guess that the grading works this way for group discussions because you can actually see what each group member contributed to a discussion, unlike a group assignment where there is usually one submission for the entire group. On one hand, there is an inconsistency between assignments and discussion grading, but on the other hand the work is different too. Now a group assignment does have a checkbox to control whether grades are assigned to the group or individually, so maybe that could be brought over to discussions. I guess personally, I have trouble wrapping my head around a single group grade for a discussion even though I have no problem with doing that for an assignment. Maybe I'm just so used to the way that Canvas works that it influences how I think about it now.
-Chris