To assess species to find out if they are of a threat to the environment would be extremely expensive, and you'd be crazy to think that the government would be handing out the cash so people can keep a few more pets, and there is no way the EPA would fork out with all the present environmental problems at hand.
So, without a risk assessment, unknowns need to be treated with cautiuon, the risk of a chameleom causeing a complete extinction is low, but it could very well put pressure on some environments or animals and possibly cause localised extinctions of some species in some areas if it can become well enough established in those areas, it really isn't that hard at all, and seeing as we don't know for sure, it would be silly just to say, 'nah, I doubt that cute little thing could cause harm'.
Allowing exotics will simply benifit those who want more animals, thats it, it won't nessicarily even stop them being kept illegally, those who aren't legible and those who don't care won't even bother applying for licences and will keep them illegally.
