A very high level of academic ability can sometimes indicate a corresponding high level of intelligence, but there's not any particularly strong link between grades and intellect. In cases where intelligent people have the desire to go on to do things which will require top grades - competetive university courses, for example - then they are likely to acheive them, but for an intelligent person who wants a career which will not require grades, there's no reason for them to acheive at school. Good grades more often reflect future desires than intellectual capacity.
For an entrepreneur, grades don't mean a thing and so a person who wants to build up their own business won't bother with them; rather than wasting time learning things that have very little impact on the business world, a potential entrepreneur would be better getting hands on experience.
EDIT: As an academic acheiver myself, I don't feel in the slightest bit cheated by people who are sucessful without doing well at school. If anything I feel cheated by the fact I've had to waste so much time in school myself to in order to get a few letters printed on a piece of paper that will facilitate my advancing in my chosen field.