In the Dallas, Texas area the "Toll Tags" are indeed tied to a particular auto as I discovered when I got given new plates. I totally forgot to update my account and was rudely reminded by getting a ticket because my tag no longer matched my (new) plates. The ticket showed a very clear photo taken as I passed thru' the booth ! I pleaded ignorance in an email and they dropped the penalty $$.
That's weird, I've swapped out my toll tag to a different car without any problem. I don't think I even entered the right license plate number, I just did what I thought it was on the top of my head.