Name one series where this is true. Any series, anywhere in the world. just one.
Fact is, some series allow two wheels outside the white lines, some include the kerbs as part of the track, so you can go all four outside of the white lines, as long as two are on the kerbs, and some, like F1, simply state that as long as one tyre is in contact with the white line, you are considered to still be on track.
F1, and many other series around the world, also have more relaxed track limit rules regarding specific parts of specific corners at certain circuits. Places where going beyond the track limits is not considered to give any competitive advantage. As
@lancia delta hf said in their examples, you should not be penalised for running wide off track in a fashion that actually loses you considerable time, and you shouldn't be penalised for one wheel going beyond the track limits. However, I won't hold this against AC, because almost all racing games have poor implementation of penalties. Pcars is horribly inconsistent too. The F1 games are the best for me in that regard, but even those can give bogus penalties from time to time.
Pcars uses the "Madness" engine, which is developed by SMS. Earlier versions of Madness powered the two Shift titles and Test Drive: Ferrari Racing Legends. The huge amount of bugs and rendering issues in Pcars is because of the in-built flaws in that engine that have been in all of those games. However, important physics components like the tyre model were completely re-done from scratch for Pcars, which is why it feels totally different, and much more realistic, than the other games powered by earlier versions of Madness.
As for which is better and why: It's totally subjective of course, but in my opinion, AC gives the player a much more natural feel for most of it's cars, particularly road cars, than Pcars does. This seems to be almost entirely because of how well the weight and inertia has been modeled in AC. Pcars does some things better than AC, and does many things AC doesn't do at all, and because of this, they are both great sims, for different reasons, and I'd suggest to anyone who owns one and not the other, to get the other.
Pcars has tons more cars and tracks, and a really fun and surprisingly deep career mode, and also has lots of awesome things like proper rules for different racing classes, dynamic weather and time of day, a very complex tyre model which, whilst flawed in some ways, is excellent in others, and includes period-correct tyres for every car. I saw someone here the other day sum it up by saying it's a "jack of all trades, master of none", and that's the perfect way to put it. It's not class leading in any aspect, but it does absolutely everything at least decently.
AC, on the other hand, takes the completely opposite approach. It's physics omit things like brake wear, temp, and fade, and dynamic time and weather, but the things it focuses on it does really well. AC has a tiny car and track list, but every track is much more accurate than the versions found in Pcars (excluding the ones that both use the same laser scan data like Brands), and the cars are mostly very accurately represented. The driving feel in AC is superior to that of Pcars, but things like the very poor (putting it mildly) career mode, and brain dead AI, mean it's most enjoyable when you just grab a car, take it to a track, and lap endlessly, enjoying the fine details of the circuit's road surface and the vehicle dynamics. The best description of AC I've heard is "Chris Harris Hotlap Simulator".
So, sorry about the long post, but what I'm trying to say is both are better than each other in different areas, and both are definitely worthwhile sims. I'm glad I have them both, and glad they aren't trying to be the same game. They co-exist perfectly in my game library, because each scratches a different itch.