To be fair, the reason that Android background apps don't drain the battery is because Google put a _huge_ amount of engineering into power management (witness the lkml flamewars about "wakelocks" as they try to merge this code back upstream).

Android also has the lovely screen showing which apps are burning the battery. In my experience, an _overwhelming_ fraction of power use is by the screen and the radio. (And by games running in the foreground, but that's to be expected.)