First of all, who do you think the government(I'm assuming you're talking about the federal government) would turn to as their experts? You guessed it, HSUS, PETA, ASPCA, etc., that want all pets to be banned. The general public has no idea what HSUS actually is, and will accept whatever they say as fact. All HSUS has to do is give their little song and dance about how reptiles are dangerous and they can't be cared for properly in captivity, and we'll be ALOT worse off than we are right now.
Secondly, you are operating on the assumption that they would make the law exactly as you said in your post. I can garantee that if they did pass a law for licenscing, it wouldn't be anything like you said in your post. Did you think about how much they'd charge for a liscense? Based on laws that states have tried to pass, it would probably be at least $50-$100 and you would probably have to renew it every year or couple years. Plus they would probably make it so you can only have a couple reptiles per liscense, or only a couple reptiles period. And do you really want the federal government to have the power to liscense pets? Think about the precedence this would be setting. If you have get a federal liscence to own a gecko or a cornsnake, whats next? Federal driving liscenses, national ID cards(they're already working on this one), federal liscenses to buy a home or car? Plus if a state or town passes a law banning or restricting large snakes or other reptiles, at least you have the option of moving, plus it won't effect most of us. If the federal law bans or restricts them(chances are it will), thats it, no more reptiles period. And they would also probably make it so they can come and inspect the caging, etc. anytime they wanted. As someone already said, it would be almost impossible to get homeowners insurance or an apartment if what you had was public record, or if somebody got the list illegally.
Legal aspects aside, its still a bad idea. If you had to be 18 to own a gecko or cornsnake, and 20 to own boas, pythons, etc., there goes the industry. At the show I go to, at least 25% of the people there are under 18, and alot of the other people are in their 20's. Who do you think buys normal leopard geckos, cornsnakes, etc., from petshops and shows? Alot of them are teenagers or parents buying them for their kids. Some kids could still get them if their parent gets a liscense, but most kids(me included) are lucky if their parents will let them have reptiles, much less spend the time and money to get a liscense. I'm 17 and I have 3 Ball Pythons, a Columbian Boa, and a Crested Gecko. I've had 2 of the Ball Pythons for close to 3 years. If they passed a law like this before I turned 18, would I have to get rid of them? And alot of people who are into reptiles got started when they were teenagers, which would mean in say 10 years, there wouldn't be as many breeders or buyers. Plus most casual hobbyists who just have a couple geckos or a bearded dragon or something can't or won't pay to get a liscense.
-----
MY COLLECTION:
1.0 Columbian BCI
0.0.1 Crested Gecko
1.2 Ball Pythons
MY WISHLIST:
A Northern Blue-Tongue Skink
1.1 nice Jungle Carpet Pythons
1.1 Hogg Island Boas
0.1 pastel Columbian BCI
A Blood or Short-Tail Python
A Dumerils Boa
1.2 Bearded Dragons
1.1 Rankins Dragons
1.3 Crested Geckos
Breeding groups of various Leopard Gecko morphs (including Tremper Giant, Patternless Albino, Blazing Blizzard, and Bell Albino).