SimBin, Kunos, and iRacing need licensing agreements because they are monetizing off of the product that they are producing. Scratch made mods do not, as they don't charge. If they did, then that's where the problems come into play. In this particular case with this Porsche, the issue lies in that the model is from Turn10. That means that for Turn10 to make that model and sell it, they had to pay cold hard money. And now someone gets to take it and say "Oh... Well you're not using this anymore. So I will. Ok? Cool, bye." It doesn't work like that.
So, Turn10 still makes money off of Forza 3 & 4? I'm pretty sure all the copies that were ever made have been sold.. Do they get any money off of crappy "game stores" like Game Stop for the unending trade-ins and resale of F3/F4? No. No they don't. That's stealing too! OMG!