If you look on the offramps (those that are not charged tolls), there are sensors over the lanes. These are meant to read the EZ-PASS and somehow determine the difference between a METpke-issued EZ-PASS and an out of state EZ-PASS. That's how the "virtual ticket system" knows if a METpke-issued EZ-PASS vehicle has exited the system.
The former ticket system used to be in place all the way up to MM 109 at the north end of the turnpike, where there used to be a barrier (surrender ticket NB/pickup ticket SB). At some point, the system was altered north of the Gray area, with the New Gloucester tolls being the new north end of the ticket system and I'm guessing at that time the Gardiner toll plaza was built. This replaced the Augusta barrier. The pre-existing barrier at Exit 103 (today's I-295, but then I-95) was changed to a flat rate toll. Then later, the ticket system was abandoned in favor of fixed tolls at all barriers.
At one point there was talk of getting rid of most of the (south of Exit 63) entrance tolls in favor of a new mainline barrier in the Saco area. That would probably have caused the METpke-Issued EZ-PASS "virtual ticket" system to be eliminated. Guess they voted against that idea. Really makes me want to switch my EZ-PASS account to the Maine Turnpike, as I could get the in-state rate. Right now I have a MassDOT-issued EZ-PASS and I get charged the out of state rate (I'm a CT resident), so there's really no benefit to having one through Mass (even though I probably use the MassPike more than the Maine Pike.
And if you don't like the $4 toll at York, you can always get off Exit 3 or 7 and take Route 1 up to Wells. However, the slog through York, Ogonquit, Wells, etc may make the $4 seem like nothing in the end.