Why is it that individuals who recieve Pell Grants don't have to pay it back?

I understand that a Pell Grant is essentially free money but I feel that it should be paid back. You have two students who have just graduated from college--student A qualified for a Pell Grant to support his/her education while student B had to take out a combination of subsidized, unsubsidized, and alternative loans. Both students graduated from college and now have a degree that will improve their chances of finding a job. Why does student B have to graduate in debt and not student A? Their parents financial situation at this point should not play a factor since these recent college graduates are on the verge of becoming financially independent. Yet, it will be student B that has the financial burden because his/her parents made too much money. Is it somehow umed that since student B came from a middle-income household that he/she has the capacity to pay off their student loans more so than student A (that is if student A had to repay his/her Pell Grant)?