Took a while to locate, but...
at least in California they are legal.
Section 24602 (a), under Division 12 of the California Vehicle Code states:
"Any vehicle may be wquipped with not more than two red fog taillamps mounted on the rear which may be lighted, in addition to the required taillamps, only when atmospheric condition, such as fog, rain, snow, smoke, or dust, reduce the daytime or nighttime visibility of other vehicles to less than 500 feet."
The section goes on to state that if only one lamp is mounted it shall be as close as practical to the left side, and no closer than four inches from the edge of the lens of any stoplamp. Also states that the lamps must be wired in a manner that allows them to be on only when the haedlamps are on and that a non-flashing amber pilot light be mounted in a location readily visible to the driver.
Hope this help clear the water (at least in CA)
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.