I mounted the sidecases myself, and noticed a "caution" box on the instruction page. This is for the factory aluminum cases.
It says: North America only: before mounting the sidecase carriers you must remove the reflectors from the original license plate holder. Mount the reflectors with the original part on the sidecase carriers. (Then it shows the reflectors mounted on the angled bracket in the lower rear corner of the holders.)
They say that if they are not remounted there, the bike won't start.
(Just kidding about the last part.)
I guess the reflectors would be more visible when the sidecases are not attached.