Santorum didn’t butt in. He was asked by Blitzer and all he did was respond with what Rush says all the time.
He uses the absurd to make a point.
Butting in would have meant he sought out someone and offered his critique without being asked.
(This blanket explanation/excuse is redeemable for use in all future Santorum blunders and is non transferable to any other candidates for explicit use, so sayeth the Lord!)