Coach explained lats Spring that he would have recruited Doug to ISU if he knew for sure that Doug would either be a walk-on with little hope of playing or so good that no one could question why he was on the Court. Mac explained that he didn't want to put Doug in a position where people would argue that the only reason he played was because he was coach's son. Very similar to Altman's reasons for not recruiting Klayton Korver.
If you follow Bix XII football at all, you have seen that very same issue play out with their knucklehead coach and his son at QB. He is not clearly better than the other QBs, but not clearly worse, either. Certainly, not at the level to be highly successful as a Big XII quarterback, so the coach and son get blasted for the son playing when the son struggles.
I think coach thought Doug could play, but wasn't sure if he would stack up well enough against Big XII talent that it would not result in a big distraction over whether and how much playing time he should get. I am sure he was confident Doug could be successful in the MVC so he was comfortable having Doug play for him at CU.
I have no difficulty understanding that rationale.