In this case, it's a stipulation of the organizers of the event. They also make you pick stuff up along the way and carry it for long distances (logs, cinder blocks, one class said they had to carry a Zodiac boat for 9 miles).

You still carry water with you, and are free to carry just about anything you want. But you will be hauling it for the duration of the event and will wish you had left every ounce at home that you could.

My packing list for my pack (which will probably be tweaked several times between now and December 14th) so far includes 6 bricks, a yoga block (keeps bricks off bottom of bag), 3 packs Peanut M&Ms, 3 Clif's Builder Bars, 3 Clif's Gell Shots, 18 Nuun Active Hydration tablets, and a Source 3-liter hydration bladder. Possibly one spare pair of socks.