I would be apt to buy a device if it was universal...or at least valid throughout a given country. But the way I understand it now, I'd need a different one for Kansas, Illinois, Pennsylvania, New York, Oklahoma, etc.

Or am I wrong...could I buy one and use it anywhere? (Assume for this purpose...one U.S. state using it in other U.S. states....)

I'd sign up in a heartbeat if that's the case. My toll usage is always in a rental car - I really don't care if The Man is tracking where I go. I've never taken a rental car into a country that voids my rental agreement and I don't plan to. (In some places, this requires asking for a certain make/model of rental car, which I do.)