Well it's not quite that it's
imposed by the system, but rather just that it's how (at least early on in the development of OS X) Apple adamantly insisted things should be done. It seemed to me at the time they'd decided a priori that app-modal windows were an unworkable legacy design model, and had then set out to prove it (irrespective of their actual case-by-case utility) by deliberately subjecting them to an unrealistic set of constraints...
Anyway FWIW I couldn't agree with you more - I never liked this behavior, and it looks more dated every time I see it.
I ask unanimous consent that it be revised so this model remains an option (for those who feel it adds some benefit), but the default is changed to having only controls (but NOT text) in SuperCard windows auto-dim when they're in the background.
Are there any objections?
-Mark