Two main reasons.
1.) Timberline is a business. To put rails/boxes out on the snow an maintain them with the park crew, cats, and salt costs a lot of money. You wouldn't walk into a restaurant and start using their stove to cook breakfast would you?
2.) Anything that happens within the ski area boundary Timberline is liable for. Since we maintain it and keep it going it falls under the ski resort, just like any other resort in the country. So when you buy a ticket, you help pay for wages, fuel, salt, metal for rails, and cats yata yata yata. You also agree to a waiver that is on the back of your ticket or pass release. So if you get hurt you admit that you knew the inherit risks of skiing when you bought the ticket.