My guess is it's something done on purpose, and which they didn't think would be a problem. Maybe the limit is intentional, but they simply set it overly-aggressively.
It *is* good to manage the lithium-based battery's temperature. Too hot, and you'll degrade the battery more quickly. And WAY too hot, and you could get into a fire situation.
But in my opinion, the system needs to strike a balance. Everything is a trade-off. Say they limit the charge current to keep the battery below 90F, and this provides 15% more battery cycles than letting it reach 100F. Some users would prefer that. But other users (like me) would prefer to be able to charge faster during the day, when I may also need the screen on.
It's a removable battery, so even if the battery life is degraded somewhat, it can easily be completely replaced if required. This, to me, should justify being a bit more aggressive with charging the battery.
Aside: I have gone back to my S3 for now, from my G4. The G4 got me paying attention to charge current. I've noticed that the charge current on my S3 is quite low, the highest I've seen (screen off) is about 700-900mA. For a 2100mA battery, that's 2.3-3 hours to charge fully. My S3 battery temperature has been as high as 112F recently, during use in the car while charging. The phone & battery are 2.5 years old; at this point the battery life is certainly not great, but it's still serviceable.