If they are to regulate, it shouldn't be based on cc's. A 600cc sport bike like a R6 shouldn't be in the hands of a noob-squid. A cruiser of the same cc's makes 100hp LESS and weighs more to boot! My wife's bike is a 650cc cruiser but only weighs 470lbs dry and makes over 70hp. Its faster than most any harley or any other v-twin "power cruiser" on the road yet might be available to a noob-squid if a law was passed mandating less than 700 cc's. Motorcycle manufacturers do not like to post hp figures but a hp to weight ratio would be a better option if something besides COMMON SENSE is needed.
I agree basing the restriction purely on cc's is silly, as is basing it purely on power to weight ratio.
Very early in the peice the ACT (Australian Capital Territory) had a learner approved scheme based on power to wieght only. This meant that a learner could effectively ride a HD Electra Glide as the power to weight ratio was quite low.
As I mentioned earlier in this thread, NSW and some other Aussie states have adopted the ACT scheme but changed it to have a maximum cc as well.
The basic premis for this is that a learner or probationary rider can ride any motorcycle up to 660cc providing the power to weight ratio is less than 150kw/tonne.
Some bikes that fall into this are the Suzuki GS500, Yamaha XV650, Kawasaki KLR650, Ducati M620 Lite and the Hyosung 650L.
Some notable exceptions are the RGV250, RS250 and a few other 250 2 stroke road/race bikes.