probably to use the power for the license plate light.
I do alot of wiring mods...
I added my own leds to the tip light for brake function ( saved $100)..and I "married" the plate light leds to the tip light connector so that I have both.
Give us a part number for the kit, so we can take a look at the instructions. I suspect Mike has the answer, however that doesn't stop an enterprising owner from adapting the wiring as you want! On an '05 that should be simple enough.
I'm thinking its an issue with plate overlap.There doesn't seem to be much space there.Bikes that use that signal bar,such as the road king custom,don't have the lower fender light.
I installed that kit on 02 electra glide , the turn signal bar has a plug in for the plate light and it uses the terminal where the marker light plugs into