Stupidity? That's a bit harsh. It's best to be amiable at this hour.Also, the "difficulty" index is just plain stupidity. Too subjective.
It's not that subjective. I believe that there is a fairly good consensus on what curricula are difficult and which are not. If you apply to a graduate school, the degree that you earn is a huge factor in your chances of being accepted. If colleges are willing to use such a subjective factor in graduate admissions, I don't think that they'd find it too ascerbic for use in determining who gets athletic tickets. The University (as are most others) is always looking for ways to boost their quality enrollment in the mathematics and sciences, so it's not as far-fetched as you might think.
As long as you don't have a huge deviation between the top and bottom of the scale, I believe that it would be fine.
All that I know is that we don't need a spike in Art History majors in order to secure football tickets.