I feel I should point out that corrupt grading is easily detectable - one can often see it by looking at a corruptly graded paper, or by interviewing a candidate who got a high grade and finding that he does not know the subject. And thus, it is not covered by the Adams quote.

Moreover, universities have a strong incentive to not be corrupt in their grading - if they let people slip through without learning the work, employers will start to notice and discount qualifications from that institution, and then prospective students will hear of this and go to other institutions instead, and then the entire institution will collapse. (It's not immediate, or perfect, and quick action at the start of the process can save the institution, but it is a consideration).