You guys are talking about fog lamps that would remain on during drive correct? If so, it's obvious why white would be illegal as someone (most) would confuse it with your intention to back-up which could make people react with some highly dangerous decisions. If it is already common, then that is equally dangerous as you don't know when someone really is backing up.

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'08 X5 3.0si - Alpine White / Saddle Brown interior
Specs: Sport Pkg, Premium Pkg, Tech Pkg, Comfort Access, Aero Kit, Style 433 staggered 20s on Conti DWS
Mods: Carbon 35 tint, LED angel eyes, GP Thunder 7500k fogs, H&R 20mm/25mm spacers, clear reflectors, gunsmoke-tinted taillights
Coded: Digital speedo, windows/sunroof/tailgate close via keyfob

X5 pics at Flickr