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.
