You can negotiate, but presumably there's a salary scale to limit one's aspirations! You say nobody puts the salary on a job advert, but that wasn't always the case. Only 5-10 years ago I seem to remember most airports did publish the salary scale in job adverts, unless they were on a fishing trip to see what sucker they could rope in at at a knock-down price.
Why are they all so bashful these days? I expect it'll change when the job market picks up and poaching season begins again.