so why don't US 575 cars have rear fog lights

My guess Ferrari build cars for each countries requirements and if they are not needed by US fed regs they aren't fitted.
Same reverse reason why US cars have the huge side markers.

 

They are not required here,and they might even be illegal under federal law as standard equipment. The feds (DOT,NHTSA) have to approve everything,and they move slowly. Look how many years (decades) it took to get the more powerful European headlights approved here as original equipment. I used to install Cibie headlights on my cars as illegal aftermarket equipment. Aftermarket driving lights have to be covered still today in some states.

 

US cars are required to have a reflector as part of the tail light system. since US cars aren't required to have rear fog lights foreign car manufactures swap out their rear fog lights for the reflectors, but not always it depends on the light designs. Some of the Audis have both. The 458 has both, my 355 and 512TR didn't.