Prom dress code

I was a bit surprised to see this in a prom dress code (Utah?) in 2012:

"Strapless dresses are prohibited unless a jacket or shawl is worn. 'Plunging' necklines are prohibited. The backs of dresses should not show more than 1/3 of the back (directly below the armpits)."

Have they looked at a Bride's Magazine lately? Or at a 1956 prom photo?

Junior Prom, May 1956, Mt. Morris, Illinois