During my last trip to WDW one of the people in my group couldn't stay for the entire week due to her job. On the day that we visited DTD she took her luggage with her and arranged in advance for a taxi to pick her up later in the day from the area by Planet Hollywood and bring her to the airport. When the taxi drove up, a policeman was right behind it with the lights on. The taxi driver got out of the car and motioned to my friend to come to the taxi. She had to verify with the policeman that she had called for the taxi in advance so that the policeman would let him pick her up.
Therefore, in order to answer your question, I would say that taxis must not be allowed to patrol DTD looking for passengers, but are allowed to pick up people who have called in advance. I don't know this for a fact, but am going by my past experience.
