In general, wulin martial artists were notoriously independent sorts who were averse to acquiescing to government authority. Nevertheless, many wulin warriors did sell their services to imperial governments...for money, for fame, for power greater than they could achieve on their own, or even just for adventure.
Why did Au Yeung Hak hook up with the Jin Empire? He never seemed hungry for things such as money, fame, or power. He already had plenty of those thanks to being the heir of West Poison Au Yeung Fung. His only interest was beautiful women, and he could have those without the help of the Jin Empire as well. What got Au Yeung Hak interested in working for Yeun Nan Hung Lit and how did he get started? Did he volunteer, or was he recruited?