I disagree with the notion that by paying a bribe for one's PhD does not add to the newly-minted Doctor's skill set and knowledge.
By being asked to pay for their PhD, the doctoral candidate gets valuable education on the way the world really operates. He is guided through the process and attains the skills necessary to successfully bribe one's way into a better paying job or financial opportunity. If he fails to complete the bribery exercise, his exposure vis a vis law enforcement is greatly limited, so he can make errors throughout the bribe-paying process that might result in a lengthy prison term if not fully practised in a setting that minimizes risk to the student. Once the doctoral candidate sucessfully demonstrates the ability and skills required to sucessfully bribe his professor and attain his PhD, then he is qualified to go out into the real world and has learned to offer and accept bribes according to established procedures kept in place to thwart undercover operatives.
This is arguably perhaps the most important skill he will learn during his education, for even a top-notch researcher will find himself ostracised in the community if he cannot complete, where such action is both customary and necessary, a bribery scenario without being detected by persons not involved in the transaction.
"That's my story, and I'm sticking to it." - bribery suspect, after denying any knowledge of bribery occurring regularly in his industry.