I don't know the ins and outs of acceptance into the B1G, but I'm positive there were academic requirements, and the University gets to enjoy the benefits academically speaking for being part of a well thought of academic conference, just as the athletic dept. gets to enjoy the benefits of a well thought of (and viewed) athletic conference.
But to answer the direst question why wouldn't the academic side share in TV revenues from all sports? The answer is simple, because 90,000 people don't watch the Nebraska - Wisconsin spelling bee.
I wish our cohorts in the B1G hadn't then turned around and voted us out of the AAU, which membership in was an admitting criteria to the B1G!
https://www.nytimes.com/2011/05/03/education/03aau.html