Not true. The Prius has a gas motor with substantial electric assist. The Volt has an electric motor with nominal gas assist above 70mph or in the event of depleted batteries.
As I understand it, at that speed, the energy lost by using the engine to recharge the battery is greater than simply using it to drive the wheels directly. Makes sense to this non-engineer.
That said, I wouldn't buy one if you (or the federal government) paid me.