“Free disposal” is indeed a very common assumption in economic theory. Sometimes this is just done in the sense described by John (a simplification adopted because the model wants to focus on other issues); other times because it is considered realistic (indeed we have a lot of CO2 in the atmosphere precisely because it is/was free to dispose of it, i.e., not subject to disposal charges for the producer). And sometimes, of course, it is not assumed at all, e.g., when the point of the model is precisely to deal with a pollution externality.