the whole case is just a lose/lose situation for everyone involved.

Separate allegations of sexual assault by Sulaimon came from two female students in the 2013-14 academic year. Both students voiced allegations publicly, but neither filed a complaint through the Office of Student Conduct or took legal action through the Durham Police Department. The students declined to discuss their allegations with The Chronicle.
http://www.dukechronicle.com/articles/2015/03/02/rasheed-sulaimon-center-sexual-assault-allegations-prior-dismissal#.VPR-4PnF-yU

if they followed all their protocols and he's kicked off the team, why wasn't he expelled?

Sulaimon is still a Duke student and remains in good academic standing.