the only thing i can think of is that some people throw salt over a layer of hay before they stack another layer on top. the purpose, as i understand it, is to prevent moisture from forming in between the bales. the guys i know who do this only do it when the hay is newly baled.
I grew up on the coast in Massachusetts, where "salt hay" is marsh grass that is cut and cured in big round stacks right out in the salt marsh. I don't know about feeding it to horses; it's prized as a garden mulch, if I remember correctly.
So, since you saw the sign near the beach, I'd guess this is what was meant. Otherwise, I'd go with Charlene's explanation.