Idk if its the same where you are or not but cars in my country DO have multiple internal tiedowns throughout the passenger cabin. Theyre called seatbelts and you are supposed to secure any heavy objects using them. Infact, if you as a driver have not secured heavy objects down and one of them kills or injures a passenger, you can get prosecuted for negligence.

Sure, i dont know the exact intention behind the law, and it being a fine does indeed serve as revenue stream for cops, but it does at least in part help prosecute drivers who are putting theie passengers at risk while promoting safe driving habits.