To "Salt" something is also to add
fraudulent value.
For example, in the days of the gold rush and land speculation, unscrupulous miners would "salt" a played-out or never-useful mine or stream with some gold nuggests and/or dust, then sell the land to an unwitting person who thought they were getting valuable property.
This has also been extended to mean any kind of bait placed into something to make it appear more valuable than it is in order to trap someone in a sting. The difference between salting something with gold or other valuable ingredient as opposed to plating or covering it for fraudulent purposes is that salting something means implanting the misleading material into the subject in question.