I think they use the salt / salt solutions places that freezing / thawing / freezing cycles are common. In the heavy snow areas it is probably a bit pointless, plus those heavy snow areas usually have less traffic than in the rust belt.
Here is an example of what 4" backspacing + sand + salt + snow looks like after a 3 hour drive (washed before I left). These pictures do no justice. I actually took them because of how the ice on the side seemed to be growing straight out horizontal, and because of how far back it went:
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Here is what wet salty roads does to you after a couple hundred miles (also washed prior). You can't see much besides the rockers, but if you look at the flip glass for example, the entire jeep has a film on it. Silver hides it from a distance. If the jeep were black, everything would look like it had a white/grey film on it:
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