This behavior was programmed into SketchUp long ago, almost certainly based on the idea that associating a layer with something implied semantics that would be lost if the layer was discarded. For instance, a CAD user would be unhappy if things identified as plumbing lost that distinction and became indistinguishable from walls.
But that was before the community came to fully understand that layers (now tags) don’t affect SketchUp’s unique behavior that edges and faces stick to each other and intersect regardless of layer/tag. So, in SketchUp a user would be unhappy to find that edges tagged as plumbing can’t be edited without pulling the edges in the walls with them. Further, in SketchUp, once edges and faces are grouped, exploding the groups is seldom the best way to modify the model.
So, there is a solid argument that the existing explode behavior is defective. But I agree with Jack and Dave that because there are workarounds, workflows, and extensions that avoid the issue, the developers aren’t likely to give any priority to fixing it.
To me this makes perfect sense when you explode a group or component with nested groups/components. When you explode and get raw geometry however I think the rule of avoiding tagging these should be prioritized. Also, when exploding into raw geometry you already lose so much of the semantics. The raw geometry isn’t an object of its own anymore but makes up the content of the parent object.
It’s easy enough to correct these things (immediately after and if you are aware of the odd behavior), but just like imports placing edges on tags it is just wrong. Now Utagged is just a tag of a different name. These are some of the gotchas built into SketchUp and LayOut that so endear us.