Before I begin...I didn't read all the responses, so it might have been said already, but...
Park crews use salt in warmer temperatures on certain snows (mostly anything 40 degrees Fahrenheit and above and on slushy spring conditions). Timing is key when using salt, if it's too cold or the snow isn't right, you'll just get straight sugar, which SUCKS to work with. Most of the time it's not rock salt or table salt like most people think of when they think of salt, but more of an ice melt salt (usually Calcium Chloride or Potassium Chloride). It causes a chemical reaction in the snow that lowers the melting point of the snow (ever make ice cream? same concept), that reaction melts the top layer of snow (VERY small layer...like millimeters) which then reacts with the snow underneath refreezing it cause the snow to lock into place. This creates a smoother (assuming it's been shaped properly...you don't have much time to fix stuff after it's been salted), more lasting jump/kicker that will react more like a jump does in the middle of winter (allowing people to carve lips, instead of it mushing out).
Some mountains/parks even has snowcats designated to salt jumps and even trails with a spreader, which is pretty sweet....cause the bags are heavy and throwing it screws up your hands (dries them out and cracks them).