Successful founders are already edge cases, and these grants won't be handed out to the stereotypical "dropped out to play World of Warcraft" population. If someone is capable and driven enough to attract these sort of awards, I wouldn't expect their (dis)taste for formal education to have much effect on their odds of success.
I think the real point here is that they are making bets on the young and idealistic kids that haven't been tarnished by years of drudgery, beauracracy, and being told that they can't do things. So they do whatever wild ideas interest them, and if any one of those kids happened to be right about something that more 'educated' people would consider idiotic, it's a massive win. Kind of like the old DARPA approach or Google's 20% time. In that respect going after dropouts would make a lot of sense, since those are going to be the types that care less about following the establishment and will be more likely to execute genuinely novel ideas.
I'm 30, and I've worked at various corporations, both small and large, for 5 years. I've never once been told "I can't do things". Not once. Rather the opposite - I've worked with people who went on to found start-ups, and I've learned a great deal from my colleagues.
The larger question is: should a person in a position of power be encouraging young people to drop out of school? It strikes me as irresponsible.