It makes perfect sense. Roberts is not an activist, he's a legalist.

The Ruling is solely on the lack of Legal Standing of Prop 8 Repeal Challenge, not on Prop 8 itself. So it's a legal issue.

I think they way it works is that, Roberts took polls and said it's going to be 5-4 with Prop 8 going down in all cases (with different 5-4). So which one do we settle on, and they chose the least ideologue decision.