It's no different from any other airport. there are automated anouncments in JFK sayin the same thing.
The reasons are split between tourist protection and $$ for the airport.
1) Authorised Taxis pay a fee to the airport.
2) Tourists have no idea what the going rate should be, amd what a taxi looks like...
3) comparing the official price to jlm and and the blackmarket price is not really fair. the prices are based on a model in which the next driver must take the next passanger basicaly wherever he wants in the country, with no hope of a return fare, so all fares fit the model. the BM JLM fares are usually taxis who took people to the airport...or the availability of a return to the merkaz is likely.