Not entirely true. For example, I usually rent out of Orlando, usually with Alamo and never use the toll/Sunpass option. The only time I use the toll road is from MCO to WDW and back, which is two cash tolls of $1.50 each way. However, for Miami, read below...
Depending upon the agency, you could decline it. As I stated above, with Alamo, you don't have to take the toll option. However, if you're going to take the Florida's Turnpike from Miami to Orlando, which is by far the best way, you can't pay cash as the Turnpike has gone to a cashless system. (They use something called 'toll by plate', which snaps pictures of your license plate). I'd recommend looking at the individual rental agency policies. Once again, I'd recommend Alamo, since they hit you with a surcharge (I want to say $3.95), but only on the day you use the tolls -- which may be just the day you drive to and from Miami. Some agencies hit you with the surcharge every day you have the rental, whether you use toll roads or not.