I don't see how they could cancel an auto policy for what you may haul in it. What if the guy makes wood stocks? Geico and their little lizard both sux.
If you read the comments, there's a rather persistent fella saying the guy is being butthurt about using his vehicle for something it wasn't intended for. Maybe you should actually call and ask them if that is their policy before canceling. Also, they no longer offer government employee insurance.