Don't forget they use UNIX model too
The basic reason that people can easily update their iOS devices is the UNIX model of doing things.
Everything is in their place, especially with app store model. So, user data is absolutely in seperate place and caches are also in that folder, which can be easily ignored.
So, update process does download that gigantic file to itunes, asks Apple "serial nr is this, user is signed on, no mysterious process running. Should I go on?". Backs up the /Users (that is the only thing), likely ignores /Caches, puts that entire gigantic image to device telling it to program rom. Puts users back to place.
Same process could easily ask for patch between certain versions (games do it for decades), use standard diff like app to create new image and verify the result with Apple and do the drm thing.
You know what? They wasted petabytes of bandwidth (and users) for a single thing. Not to allow unencyripted os image to be stored in a real pc/mac.