but I don't get 'pre-college grads'. i can't think of a single mac (or macbook) owner I know who is a non-adult (I'm presuming you were meaning to indicate a youth-angle, not education level). I'm sure there are college-age mac owners, but all the ones I know are adults, working for businesses (or themselves as a business).

it's also easier to support just one platform than many.

"there's riches in niches" may be the general answer.