when you add slat, or any mineral to the outdoors you have to keep in mind, A LOT of how animals react to it, is based on what minerals they have access to or not

if they DON"T need salt (X MINERAL) in there diet, most will ignore it, if minerals are missing or more rare, in the area your at, they might be more attracted to it
which is why I said, DON:"T expect a LOT of attraction to a SALT Block
I highly doubt ANY wild healthy animal will over dose on SALT just cause its out there
so I cannot see it ever harming them

so I see NO harm here, now if legal or not., that might be another issue, NO clue where your at to check more into that side of this