Just for fun, swap the the resistor from +5 to the input pin to a 10k or 20k ohm resistor and see what happens.
My understanding is that most of these detectors are phototransistors which begin to conduct when (ir) light hits it.
Thus, this isn't really a voltage divider -- it just acts like one. What is happening is that light strikes the base, the transistor begins to conduct. The collector of a transistor closely approximates a current source. The current flows through the resistor and Ohm's law dictates a voltage drop across it, which lowers the voltage at the connection of the R and T.