http://www.leg.state.fl.us/Statutes/index.cfm?App_mode=Display_Statute&URL=0300-0399/0316/Sections/0316.074.html appears to be the regulating statute for Florida.
The applicable section in the statute reads:
(2) No person shall drive any vehicle from a roadway to another
roadway to avoid obeying the indicated traffic control indicated by
Based on another Jurasdiction (Germany) the main criteria for when seat belt are not required seems to be
that peaple are allowed to stand during transport
The UK Guidance Seatbelts on urban buses of 2010 confirms this with:
Since 1 October 2001, seat belts have been required to be installed in each forward and rearward facing ...