I'd be surprised if it was for any other reason other than to change up the looks from year to year. My 12 XF LXR has non-painted, 2013 was painted now 2014 non-painted but with different graphics than my 12. Gotta change up the looks from year to year.
Painted or non-painted, wouldn't make much difference for me. The non-painted aluminum is great for guys who like polished tunnels. Black paint would hide any light scuffing, etc.