From a legal standpoint, modders often operate in a grey area. While they create the 3D models and physics, they are often using the Intellectual Property (IP) of car manufacturers (like Ferrari or Porsche) without a license. This makes the "copyright" of a mod difficult to enforce legally, though most community members respect the "pay-per-work" model to support continued development.
Why? Because they have seen the "pirate mod" chaos of the original AC.
The prevalence of pirate mods has created a rift in the sim-racing community:
The Assetto Corsa community thrives on passion, but the pirate scene hurts it in specific ways: