Actually, the upgrades for the iPhone 3GS have gone up to iOS 6. That's their 3 version upgrade policy, implicit or not. The "limited" nature of their updates were originally surrounding hardware that was introduced in later iterations of the phone.

But core upgrades happened regardless. 7.5 to 7.8 is not a full upgrade. It's only the start screen. Perhaps some tweaks to IE. For what it is worth, it's a half-*** update. The Lumia 800 enjoyed a full upgrade to 7.5 from 7.x (can't remember the exact number). And from there, it'll get the aforesaid half-*** update to 7.8 but never to 8.0.

That's horrible no matter however you wish to spin it.