Presumably to avoid liability issues associated with representing the loans as investments, particularly relieving themselves of any obligation to chase missed repayments on behalf of creditors.

In some respects, having the up-front option to to waive any right to withdraw repaid loans might be better for people who actually have no intention of doing anything with repayments other than loan them to other Africans, assuming charitable donations whose use and re-use you're continually able to influence can still be legally tax deductible.