What's the difference, Amateur and Pro?

I could not help but notice this from Kawasaki PR about Stewart:

His recent 125 Eastern Region AMA THQ Supercross Series title was just one among many that the affable racer has won since beginning his on-track career at the age of four. The Haines City, Fla., resident signed his first contract with Kawasaki in 1993, when he was seven years old, and he will soon pass all of the sport's other 125 class competitors in total victories, all while riding his Kawasaki KX125.

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I may be off mark here but how can these kids sign a factory contract that early and still race as an amateur? Shouldn't they create Junior PRO classes and leave the amteur classes to the true amateurs.
Also about Alessi running a CRF450 at Loretta's. Should there be an Amateur Open Class? Sure sounds like a contradiction to me!