In some situations removing imports if the class does not compile results in a long cycle of errors when using a combination of alt+enter and ctrl+shift+s to fix a compile error. Hitting ctrl+shift+s is a reflex when I have compilation errors because it triggers update and recompile of related modified classes.
However, with the current implementation you often end up fighting with the editor because it removes required imports because of the compile error (they do not look like they are used, you temporarily remove reference to them hitting alt+enter and selecting different fix, ...).
Yes you could selectively save other files but it makes the whole process more complicated.
Modifying the "on save" would make this feature more compatible with other IDE "automatic" features.