According to this
article, dated June 1, 2023, ND has only recently been a member of the AAU, less than one year ago. The B1G has allegedly been pursuing ND for years, if not decades.
The article from Bleacher Report that I quoted from earlier, says that Perlman announced the ousting on April 29, 2010. This
article states that NU decided to leave the Big XII on June 12, 2010. Even if the dates are off slightly, it's pretty obvious that, at the very least, NU was admitted to the B1G as the process for booting NU from the AAU was in progress.
I'm not arguing against joining the AAU. I'm happy NU is currently trying to get back in. I'm simply saying that the B1G took in NU even though they weren't a member of the AAU and, as such, lack of membership isn't a valid reason to boot them from the conference. Frankly, the idea of kicking NU out for any reason is, IMO, absurd.