Because that would have proved that the porn was fake. Or maybe you were sleeping that day in STAT513, when we went over the Minion/Dusek Theorem of 21....
It's simple, really... If Westerfield were truly innocent, some of the evidence against him would have been irrefutable. Since all of the evidence has gaping holes in it, he must be guilty.
Isn't it obvious? Good thing you aren't in SD law enforcement, otherwise the implications of DW's cooperation would have gone unnoticed.