Hmm. What if your image wasn't originally intended to be commercial, but the opportunity later arose to make money from it? Just ask permission, you'll probably say. But what if your image involves a LOT of freestuff, say a street filled with parked cars? Are you suggesting that the artist track down and contact the creator of EVERY SINGLE CAR? And since those cars are probably based on real models, would the modeler really have any rights anyway, since they can't exactly claim to hold a copyright on a model of, say, a Ford Mustang? Just a hypothetical example here, but one that could certainly apply to SOMEONE. Just model it yourself, you say. Fine, if your just doing a couple simple objects. But what about the street scene? One lone artist modeling every car, every building, every streetlight, traffic sign, trash can, candy wrapper on the sidewalk, etc.?