In my view it was actually quite strange of UNIX that it by default let arbitrary user code stay around unrestricted after logout.
Hell, I'm no guru but I know the answer for this - it's so you can run long tasks without having to tie up a limited terminal in the lab. Log in, start your task, log out and go for beer while someone else gets use.
Now, in this world of "everyone has their own computer and terminal", it's not as necessary a feature as it used to be. But that hardly seems a reason to arbitrarily flip the default.