Can anyone think of a reason that I cannot power down this receiver by driving its Vcc directly from a microcontroller pin?
Microcontrollers usually have a limited amount of current that ALL the gpio pins can soure, notjust on each individual pin. You need to check the total current that ALL the gpios are going to output.
Hint: the amount of current microcintrollers can sink is higher than they can source.
To control the ir receiver by sinking current, put the gpio in open collector mode. In effect, the micro is switching the 0V on/off instead of the power.
This may not be an problem, do the check like a good engineer
