We have two topics going at the same time here. I'm afraid you've mixed them up.
Topic 1: GT3 is programmed to always second-guess your steering input. If it didn't, driving with the DS2 would be nearly impossible. However, it has one flaw -- it seems like it doesn't always let you use your car's full steering lock. Here's a crude illustration:
The reason why it does this is most likely to avoid excessive understeer, because there are few instances where you would be using a car's full steering lock, and most of them are at low speed. However, when you're in the middle of a massive sideways slide, you want as much countersteer as you can get. GT3 doesn't give it to you, so you spin out.
Topic 2: In real life, it isn't very hard to throw a RWD car
so hard into a spin that even full-lock countersteering won't save you. In my opinion, GT3 simulates this pretty well, and GT4 fails miserably at it.