There are still HP restrictions - nobody is going to bother getting a Civic and modding it to 'just' 1000hp. Even if they do, it'll generally be fairly obvious and it's honestly not that difficult to kick them.
1. They're not purposely allowing hacks. They can't fix or are having difficulty fixing them for what ever reason. It might be a flaw with the game.
2. I doubt people would hack GT6 on day one. It takes a while for them to figure everything out.