adding columns should affect only local document structure
Adding a new column in a subfolder adds the same structure to the whole Sharepoint. I cannot in my wildest dream imagine how anyone can think this is a good idea. I cannot think of any (actually good) file structure that works like this. This is one of the worst design choices I have encountered. I mean, Microsoft in general is truly bad at most user experience, but this just goes beyond what I can comprehend.
At least allow it to be a choice if columns are global or local.