I don't understand why the Santorum people didn't force Romney off the ballot here. It's all about delegates at this point.
It looks like this all happened back in January, way before the campaigns thought they'd have to worry about every single delegate. Santorum's people probably thought they'd look horrible for forcing Romney off the ballot because of a technicality and maybe prevent Santorum from overtaking Gingrich; Romney's people probably thought they'd have the nomination locked up by the time Illinois came around anyway.