Differentiate between several non-cash programs: (1) rental car programs that charge tolls to your credit card (the card used for your rental bill), based on actual usage plus a daily or weekly service fee, (2) rental car programs that provide unlimited usage of the toll roads for a set daily or weekly fee -- as far as I know, currently with Dollar and Thrifty for use on Sun Pass roads only, and (3) inadvertently using a cashless lane and being fined or billed after the fact.
The toll roads photograph the registration plates of any vehicles that pass through the cashless lanes without having a proper transponder to charge an account. The registration number is compared to a database of rental car registration numbers, and a match bills the toll to the rental car company, which then collects it from the renter with the service fee. If the registration number isn't in the database, then a bill is sent to the owner of the vehicle, along with a fine. There are some exceptions, depending on the toll road operator.
In the Orlando area, there are two toll road operators, the Turnpike (Sun Pass) and the OOCEA (E Pass). I have an E Pass transponder, which works reciprocally with the Sun Pass. Having the transponder or a billing arrangement through a rental car company can be convenient when there are long lines at a cash toll booth, which is occurring more frequently as the operators change over to cashless and only staff a single both in some places. However, I personally wonder about the value of the unlimited use program, because it is unlikely that a typical tourist will use the toll roads enough to justify the fee, which is currently about $7 a day or $37 a week. Maybe that's best for somebody on business, who is calling on places throughout the area.
In the long run, cashless is the plan. Residents can have transponders, and rental cars can be billed. However, for those visitors who drive their own cars to Florida, there will be a windshield card with RFID circuits that can be purchased at gas stations, etc., and charged with a prepaid amount, similar to how a fare card works on mass transit (for example, Washington DC Metro or an Oyster Card in London).