Salary is generally understood to be compensation for the work you've performed; attempting to redefine it that way would require them paying considerably over the normal unencumbered market rates. Those kind of deals do happen but they're rare: people with very specialized skills or connections do get paid for exclusives and the arrangement is formalized up front and comes at a hefty premium, not hidden in what otherwise looks and pays like an ordinary job.

As for “relatively rare”, most claimed trade secrets aren't as sensitive (or hard to independently reinvent) as people like to think. When the cost is subsidized to almost nothing, people will claim everything is critical but as soon as they have to pay for it, they'll focus on things which actually matter.