Not so sure that "scam" is an appropriate term for a dispute over a toll.
Like all car companies, Dollar is pretty explicit about toll road do's and don'ts BEFORE one reserves their car. Of course, all rental companies offer an option to pre-pay for use of a cashless toll receiver and these devices are admittedly a bit pricy. The problems one might encounter with toll roads depends upon the state and the specific toll road. For example, there are toll roads which either use electronic systems exclusively or they have un-manned exact change lanes. When I rent a car in Houston TX there was one toll road network which used electronic devices exclusively and Dollar was pretty explicit about avoiding these roads if one does not have a transmitter.
We've used Dollar for some 14 years in the vicinity of West Palm where our rental agreement is pretty clear about having a Sun Pass or other device for paying along with a warning about driving through a lane without one.
Straight from a phantom Dollar reservation I create for a car out of PBI:
Please note: There are toll roads within this area that have converted or will be converting to all-electronic tolling or cashless tolling. In these areas there is no option for a rental car to stop and pay the tolls in cash. If your travels will take you on a toll road in an area where there is no option to stop and pay cash for the tolls, PlatePass All-Inclusive Tolling is the perfect solution. If you choose not to select the PlatePass pre-paid tolling option and travel on one of these all-electronic toll roads, and do not have your own tolling device in the vehicle, the applicable toll will be paid for you and an administrative fee of $15.00 per occurrence with a maximum of $105.00 will be charged.
Judging by what the OP says, Dollar invokes the $15 administrative fee plus the toll and eschews charging the maximum $105 fee. Lacking a receipt, there is very little one can do in this situation, but I hardly characterize this as a scam. Likewise a search on the Florida Turnpike has explicit mention of avoiding SunPass or other device-expedited lanes if one does not have a transmitter and to follow signage as the tpk always has one human-manned toll lane.
Not sure what watching a GPS does for driving through a toll plaza or the relevancy of this comment to the situation as you either figure out how to follow the signs where the cash lane is or you go through the wrong lane and the vehicle is flagged for a violation and charges the customer. My other thought was that the OP went through a cash lane where the coinage may not have finished dropping into the basket and a violation camera goes off, but the OP insists they had human toll contact ( suggesting possible human error by the toll-taker).
In any case, I simply do not buy into the blame Dollar logic of this post.
barry