Damaging someone else's vehicle is considered a motor vehicle accident wether you run into them, drop something off the top of a load, or throw a rock and smash out a windsheild.
Had the driver not stopped to exchange info I would be required to submit a police report. From there police would have to track down the driver and interview him. If it comes out that he knew about rock and windsheild but didn't stop anyway he could be charged with leaving the scene of an accident. The onus then is on me to prove that the rock came from your truck, made easier to do when you are missing a mud flap, which means a day in court for both of us.
So which would you rather do, pay out $100 for a new windsheild and get your flap fixed or have the cops poking about your bussiness and eventually taking a day off work for a court appearance?