"I think people we come in contact with sometimes get into trouble and it's not necessarily anything to do with us. So I would treat it like another case that is before the courts."
Harper is both a workaholic and somewhat introverted. We know what he does with his time. The crimes of which Nolan are accused were not openly flaunted, which is why they've come as such a shock to so many.
It'd be easy to try and tie Harper to this directly (especially in light of how he apparently didn't know a lot about what his own staff were doing in his interests) but to do so would be crass, cheap politics of the sort that gotten people so cynical.
Rob Ford, on the other hand, knows full well what the likes of Sandro Lisi get up to. Ford also, drunken stupors or not, walks himself into compromising and periodically illegal behavior.
You can't judge a person by the dirty secrets of their friends, but you can tell a lot about them by the company they knowingly keep.