It's Florida law that no one is allowed to charge extra for one-way rentals within the state. The reason why specialty vehicles might not be allowed to go one-way is due to computer-calculated supply and demand.

For instance, if you want to rent a generic Grand Marquis from Orlando to Tampa, chances are high that somebody else reserved the same car class for pickup at your dropoff city. Someone may take an Avalon from Miami to Orlando, so statistically this random circulation of the corporate-owned fleet will eventually cancel out and result in nearly constant inventory levels of each car size at each location.

The problem arises with specialty vehicles, such as convertibles, cargo vans, etc., where demand varies unevenly. As such, there might only be a few of those vehicles allotted to the area, and they need to be shared among several locations. Thus, those locations can't afford to "lose" the car, because they might not get another similar one back later and their inventory levels would keep being depleted.