Setting a floor for piecework pay isn't the same as making it illegal. There's still payment according to widgets produced in all other cases. I agree that it does change some of the incentives for piecework, and perhaps you could argue that it's effectively prohibited, but piecework per se is nevertheless perfectly legal.
The point is that if you're receiving a minimum of the minimum wage, then by definition you're getting a wage. If you're getting a wage, then you're being paid per hour. If you're being paid per hour, then you're not being paid piecework and OP owes his/her employer another 38 hours per week.
If your piecework is enough to make more than 40 hours at the minimum wage, then you don't owe the employer anything because the calculation for your pay is now decoupled from the hours that you work.
