I still wonder why it was decided to do it that way. It makes much more sense to do it the AutoCAD (and probably other programs) way where exploding a block puts Layer 0 and objects on other layers back on the same layers they were when the block was created. Based on the number of questions I see around here, the SU way creates many problems that would be avoided with that other approach.
The explode/tag thing isn’t even all that consistent. For example, take a group tagged “body” that consists of 4 groups with a “leg” tag and one group with a “head” tag. All of the geometry in each of the groups is “untagged”. If you explode the “body” group you get 4 “leg” tagged groups and 1 “head” tagged group. Explode any of those groups and the raw geometry contained in them suddenly isn’t “untagged” anymore. I can’t think of a benefit to that happening. Oh well, IIWII…