RL,
Here is why many people suggest that the route taxis are meant for the locals and not tourists.
The tourism industry has been growing since it started in the 1890's. The first tourist association was formed in 1910 (Jamaica Tourist Association). The first government agency was founded in 1922 (Jamaica Tourist Trade Development Board). In 1954, the government reorganized the Development Board and called it the JTB (Jamaica Tourist Board) that we know today. JUTA (Jamaica Union of Travelers Association) was formed shortly thereafter to serve the increasing transportation needs of the ever growing tourist industry.
The government has known since the 50's that the Jamaica Public Transportation would not be able to handle the various and growing needs of the tourists, thus the need for JUTA to exist. There are now other unions serving tourism needs in addition to JUTA (JCal and Maxi to name two) as the needs of the tourism industry grow.
So, JUTA and the other tourism taxi associations were designed with tourists in mind, not the island route taxi system. The local taxi system was designed for the local populace, although anyone on the island is free to take it. In fact, it is illegal for a driver to discriminate against anyone wanting a ride. And please note that the US Embassy recommends that their American staff avoid public transportation.