Full-time employment is also a requirement (and I believe 20h per week is not considered full-time in the visa context) so either they've found a way around the law or the USCIS is going to bust them.
I don't think full-time employment is a strict requirement of the H1B visa. It's just that most companies wouldn't go through the hassle of H1B sponsorship (and it's a huuuuge hassle for both the employer and the employee) for a part time employee.
I stand corrected, it's not a requirement per se, it's just a part-time H1B still has to meet the prevailing wage requirement, which is established for the full-time job.
