Easy for many reasons:

1. A fat carburation limits power, but skyrockets the fuel consuption. Great, he doesn't has high output, but a tank could last barely 4 minutes or 4:30 (or... less). I don't know if you have seen that the main problem is facing people now is not critical carburations, or other problems (engines have evolved a lot in those terms), the main problem is the burning rate of the fuel, and the tank remains the same... 125cc.
2. A more powerful car and a newbie overhelmed by the output of those powerful engines could chicken and not open full throttle on the backstraights, so the mixture starts to self lean due to partial throttle, so the car leans and continues accelerating (side effect) alas, the leaner the mixture, the hotter the engine gets, so saw some pretty meltdowns of really expensive engines by 'part-throttle chickening at the backstraights'.
3. A more powerful, because their higher output, their life expectancy is lesser, so doesn't matter if you drove it in 'balls out' mode or 'chicken' mode, he has a life expectancy.
4. Because when that driver wants a more powerful engine, probably the market made some engines cheaper and he will save some bucks on their brand new engine.

Made the test. A friend got a MRX-3 and the shop sold it a JP engine. That engine was a beast and he was unable of driving and pulling the trigger to the max, so engine suffered. One day, exchanged their 'black beast' for one of mine 'old and trusty' RX21 engines and he improved a lot, because the engine had a lower power output and was easier to drive, so he was using more throttle more time. He improved on fuel time and lap time, so more power and fatter carburation at the same time on a newbie is a big NO-NO.