Installing motion lights on a fixture with a dimmer switch
The dimmer modifies the sine wave and the fixture may not work. many dimmers  use silicone controlled rectifiers and only 1/2 of the energy wave is passed on even at full on. Only having half wave to work with can keep the fixture from working or may over heat and even damage components in the motion light.
Because the manufacturer states to not use this fixture with a dimmer it would be a code violation to do it.