I will add in that Kate Upton made a huge splash (allow time for mental image) in 2013 with her body painting SI edition - I am sure JR has extra copies if anyone wants to buy one.
That was right around the time of this shooting, and I imagine both from the director's point of view as well as any of the cheerleaders that might have wanted to have a modelling career and portfolio, the topless body painting was a "IN thing" at the time.