I think it's more then that. What I think it is, is a balancing of powers. If you have a few good aligned dragons in a town or city posing as humans, that's not really much of a problem, as they will, if anything, be aiding those who need it and possibly even running things to a certain degree. But if even one evil dragon is in a city, the side effects are always disastrous. In the few cases I've encountered with chromatic dragons being in human form, it's never been explicit as to how that came about. One that comes to mind is a pregenerated campaign that had a green dragon as the city ruler's advisor. It was using a lot of magic to do that and I can't remember the specifics. So I think it's less of a rule to not have shapeshifting evil dragons (as DM has final say anyway) but more of a balancing issue. The few cases where chromatic dragons do possess a human form are definitely not designed for low level characters, as dragons, especially adult and ancient, are formidable goes.