> The theory put up is that your behavior in university, before you get the diploma does matter.
Ok, but why should it?
The diploma doesn't certify that you're a pleasant person, have good morals or follow the law. It means that you passed some exams and are expected to have some competency in your subject. The only reason to doubt that this is the case after already giving someone a diploma is cheating, which means they didn't actually pass the exams. Other misconduct during working on the diploma has nothing to do with the qualifications for the diploma.
University used to prove that you are an upstanding citizen capable of holding public office and doing that well, honestly and fairly. Not destroying stuff and following the law is part of that. That's the reason it's required in Turkey. It was very much part of the fabric of the Roman Empire, and when muslims conquered the eastern roman empire, they didn't conquer it to destroy it, they just wanted its money and power. So they kept everything intact, except the head of government, and they kept the rules about public office requiring a diploma. They went so far as to repress islam, even keeping most of the social progress that "the prophet" swore to destroy (such as the abolishment of slavery, which they mostly kept abolished, at least in Turkey itself) to try to maintain the economic power of the empire, which of course didn't work, but ...
For that purpose it makes 100% sense that good behavior is a requirement of getting a diploma.
That's the reason the Turkey situation is what it is (of course Erdogan has already destroyed public institutions in Turkey, during the Erdogan-orchestrated "coup" against Erdogan). There, it's being used as an electoral weapon, and that's not what's happening in the US.
The American situation is different, but some parts remain. A diploma is not meant to just prove knowledge, it is meant to prove that you can be trusted to (help) organize a particular part of society, and that includes things like behavior standards. A lot of this, such as loyalty oaths (as in multiple, most famously the hippocratic oath, but certainly not just that one), the requirement to be accepted by currently important public servants (e.g. there was a time in Europe that to pass lawyers needed to present themselves to a judge from the supreme court and survive whatever test he wanted). There were globally mandated subjects, as in for every university degree (philosophy, state structure, religion, law, rhetoric). Those were not just mandated courses you had to take, you had to pass. 100% on everything, but fail religion? Tough, no diploma (one famous example of that was Einstein, who made it a sport to fail religion class, and was as an exception granted a diploma anyway)
Ok, but why should it?
The diploma doesn't certify that you're a pleasant person, have good morals or follow the law. It means that you passed some exams and are expected to have some competency in your subject. The only reason to doubt that this is the case after already giving someone a diploma is cheating, which means they didn't actually pass the exams. Other misconduct during working on the diploma has nothing to do with the qualifications for the diploma.