It looks like this is due to the Hot Climate Version being part of the spec for the US cars.
When you change the model code from 1J71 to 1J73 (US spec) on the configurator you can see the lower grille changes - the silver slats disappear and additional openings are visible. (Because of the openings it might not be possible to simply retrofit the slats depending on how those are attached.)
A similar thing happened with the F30s in the US - the Sport Line bumpers have an additional cutout in that area and the M-Sport cars don't have fog lights.
