That is because our laws are far different than yours, just as how our government is setup. You don't get a federal drivers license, you get one from a state. Vehicles are not registered federally either, they are registered at the state level.
Does your country also force new drivers into an econobox or could they drive a supercar? I find it odd in that some countries have GDLS's but it is only for motorcycles. The ones that use displacement as the sole basis is also quite odd given that displacement doesn't mean power. If that were the case, HD's would be outrunning everything.
Firstly - good idea on creating a separate thread.
There is no federal licence in Australia either - it's by state. However certain aspects of automotive licensing applies nationwide. Such as Heavy Vehicles.
If you read what I said: motorcycle licensing is based on displacement AND power/weight ratio.
, so yes using displacement as the sole basis is odd and that's not how the government does it. There is no federal registration here either.
Certain 2-strokers are also banned as an exception such as the NSR250 despite them being within displacement and power/weight ratio - reason is due to the difference in power delivery compared to 4-strokes. Probably based on evidence that young learner riders try and find loopholes, get something they eventually crash due to inexperience or unfamiliarity.
As for your second point; funnily enough learner drivers can drive a supercar since they would be supervised by a fully licensed driver in the process. This is another potential "loophole" but in practice it rarely happens because a $400,000 Supercar is obviously worth
a hell of a lot more than a Superbike despite similar performance. How many parents would gladly handover the keys to a Ferrari to their son whilst they are learning? That being said I remember an incident where there was an accident with a learner driver behind the wheel of a Ferrari. Idiocy like this is what causes more paperwork and more regulations.
Probationary drivers are allowed to drive by themselves and hence are subject to restrictions - so in other words they can't drive whatever they want. They are restricted depending on power/weight ratio of the vehicle.
Leaving aside government regulations - the bigger question would be more along the lines of common sense. Is it reasonable to allow an inexperienced rider to learn on high powered motorcycles? The answer is a no. It's down to probability:
Likelihood of crash
1) Inexperience + high-power motorcycle
2) Experience + high-powered motorcycle
Out of a sample of 1000 riders in each group, how many more do you think would be crashing in the first group compared to second?
Seeing as most of the world follows graduated licensing including countries where most Superbikes come from...Japan and Italy, I'd say they are getting it right.
Oh...and Youtube videos are evidence enough for me that inexperienced riders should be nowhere near motorcycles north of 600cc.