> Applied this consistently, it's clearly a design decision.
Perhaps the design decision assumes the separation is due to a divorce? In that case, keeping them separate is a reasonable decision since neither party would want to be forced to endure further shared memories on any device.
Adding my own experiences with iTunes-wiped devices and music-collections...
The only consistent thing with Apple and music/services is them accidentally wiping or deleting your data from both your devices and their services.
Applied this consistently, it's clearly a design decision.
What kind of company designs things to behave this way? Really?