Mike raises a good point. And this entire thing was MY BAD, I thought I was banned cause I didn't realize the other thread was locked. MY BAD.
For example, in the iRacing TOS, as well as Trading Paints (the program used to display custom car skins), it explicitly states you cannot use logos or artwork on your design that you don't own the rights to or have not been allowed to use, punishable by suspension or outright ban.
This makes 95% of all car skins you see on iRacing, against their own T.O.S. If iR were to enforce this rule, the sim would be a ghost town aside from the few people who haven't installed Trading Paints yet.
On the flip side, over at NoGripRacing.com, there are several mods for NFS Shift that feature car models and sounds ripped straight from Forza 3. However, the creators included a disc check during the install process that requires you to have a copy of Forza 3 in order for the installation to proceed. As far as I know they have not run into any legal troubles doing so.
If you wanna ensure the right people are credited for their work, include a disc checker in the installer. Problem solved. There just aren't enough mod teams to go around, so using a combination of conversions, ripped content, and original content, is the best way to let the catalog of tracks & cars grow. Those concerned about AC consisting of only ripped content need not be worried, as many scratch-built mods can be seen in development over in the mods section of the AC forum here.