It's to avoid unexpected problems.
The layers bar was originally made to work with the layers of the current layout. Showing layers from other layouts and also being able to edit them normally will most certainly run into all kinds of subtle problems. Showing them and not allowing to edit them is a middle ground, so you can at least see if there are layers from another layout being shown in the current one.
Global layer items have a context menu option to open the layout of the original global layer, so if you want to edit those layers, you can quickly find the layout they belong to.
As for deleting the layers with a -1 index, I think that should be ok as in most cases once you make a layer global, you don't care too much about what it was showing previously.