TB and Lilo, you're both right in terms of Vito's point of view.
However on a meta level, Vito has been described as a loving father. Why would you put your son who is weak and soft, a man who is absolutely not made for being a bodyguard or an entrepeneur into such circumstances?
Vito had certain plans for his sons instead of giving them freedom. They had to fail. Vito had earned so much money, he could have easily let Michael study math.
Instead he let them continue his own criminal life.