The reason NU doesn't understand other than grads being fans is they've never seen it happen before.
There are very few college programs that can garner a significant fan base outside their alumni.
In the south east, it was easier in the past, I believe, because professional football did not have a significant presence there for quite a while. Atlanta and Miami started in the 60s. Tampa came in the 70s, then the Titans in the 90s. Growing up in Tennessee in the 1970s, college football was the biggest stage around. It's popularity far exceeded professional football. The same is true for some areas of the Midwest. But in most every case, it is the big state school that wins the day.
A few other schools have a particular appeal that draws fans for one reason or another. For example, Notre Dame was long the official school of every blue collar Irish Catholic in the United States. But, schools like Northwestern in a large city with an NFL franchise don't stand much of a chance to draw a whole lot of love.
These days with dozens of CFB games being televised regionally and nationally, it's easier to happen. Fans can consume a football program with little or no exposure to the other aspects of the university. Obviously, the more popular the program and the greater exposure it gets, the more likely it is to draw these kinds of fans. This is likely why you see the dominance of "Roll Tide" nationwide. For the longest, it seemed like it was Florida State that was the trendy school.
I don't necessarily agree with NU's stance, but I do understand where they are coming from.