The Contract specifically prohibits employment by anyone else without the written consent of the Employer.
True, but some individuals have applied for, and received permission. Possible examples would be starting their own business, or being "employed" in some capacity by their wife's business. Or, as per Basil's post, being a male escort.
That being said, some of the iCadets on C Scales with a family might seriously need to get a second job to supplement their income.
I can just see it now. "Sorry guys, do you mind if I take 8hrs of controlled rest in the seat as we fly past Kunming? I've been hard at work at my other job all day, and I'm just beat."