It's because of a technicality in how Arthur's school district handled the situation.
Initial eligibility requires two things:
- High school transcript that shows you took the required core classes and had at least a 2.3 GPA in them
- Qualifying SAT or ACT score
If you have both you're eligible; if you don't then you are not initially eligible. If at any later date, you lose one of those (i.e., Rose), then this means you were never eligible in the first place and shouldn't have been playing.
The problem with Arthur is that, while his high school games were vacated, the school district made an explicit decision to leave the class and grade on his transcript. So while we know the class was a joke, Arthur has an official government document that states he took the class and passed. That ties the hands of the NCAA.
If the NCAA were to have declared him ineligible, that's an instance where there is a very strong likelihood that you could sue the NCAA and win.
The other thing people overlook about Memphis is that they were informed of issues very early on and chose to play Rose anyway. They took a gamble and got burned, but it was a calculated decision. So, it's not accurate to view Memphis as some innocent victim that was blindsided by this.
As much as I love Calipari and dislike Kansas, I would have made the exact same decisions as the NCAA in both instances.