Usually the salt piles occur while they are trying to make sure the spreader is on/working before they drive off.

I live on a mountain near a park where they stage the truck before salting.

I often drive my own plow truck up there w/ a shovel in attempt to collect the “free” salt and put it in my salt spreader for my driveway.

I don’t think anyone would care if you do the same, as the employees will never be able to shovel it back into their giant dump trucks.