Don't be confused, these people weren't idiots, it's just that back when most of these animals were introduced little or no thought was given to the possible consequences because those sorts of concerns just didn't exist.
We now live in a much different society and are mostly wiser than our fore fathers.
It wouldn't be hard to intoroduce a workable system in this country, when they first discussed limited legalisation in Victoria quite a few years ago now one of the conditions was no breeding, sale etc. Now although this seemed a good idea to the law makers all it resulted in was most of the keepers with exotics didn't apply for a permit and carried on breeding and selling. Only a hand full actually came forward and it took literally years in some cases to get a permit with no guarantee when you applied that you would get one, as I said very few put their hands up for that one.
Of course there is a small risk of disease BUT as I keep saying these animals are here now and in big numbers, make them legal and have some sort of control over them rather than the way it is now.
There are species which pose next to no threat to anything in this country, namely land tortoises of most types, chameleons and lots of others. I wouldn't expect or push for a carte blanche approach where everything was available but there must be some middle ground.
JMHO.