Someone under 24 ist only restricted in power, so he can buy a bike that makes 80hp and restrict it to 48hp.

With the cbt you’re both restricted in power (15hp) AND in physical engine size (125cc)

“Restricting” a bike to 125cc would mean that you’d take a 300cc bike and changed the engine. That means, that you’d have to register it as a completely new bike that hasn’t been tested beforehand. Then you’d have to change to the engine and re-register it, paying the same visit to the inspection. And it’s not even sure, that the bike would pass that inspection. That would be a lot more if a hassle and way more costly than to just buy two separate bikes.