To what extent can a Home Owner's ociation sue and fine you?

A friend of mine lives in a small private community that just seems to give them hell. There's a group of people that walk around every morning at 7am with clipboards and look for things to write people letters about. For the longest time they wanted to sue them for having pickup trucks in the driveway. Mind you, nice trucks 2001 and 2002 and VERY well taken care of, not beat up work trucks. One guy lives on the opposite end (doesn't even have to drive by their house to get to his) threatened to sue until he was broke to get them to move. So now the 2001 goes in the garage and they got a letter from General Motors saying the 2002 is considered a luxury family vehicle. The latest thing is they are threatening to fine due to "too many unauthorized vehicles". Did I miss something? If your driveway can fit 10 cars, you can't park a few friends cars that are visiting or whatever? My experience is much different. I live in a H.O.A. and they worry about important things like repairing the roads and things... not suing over having a pickup truck in your driveway