I don't think it is fair to be critical of the U.S. women for choosing to wear bikini bottoms in beach volleyball. And yes, the women "choose" what to wear, it isn't mandated. The U.S. team yesterday chose to wear bikini bottoms. The Netherlands team that they played chose to wear tight fitting full length bottoms. The U.S. women seem to be comfortable in the bikini bottom. It sure hasn't hurt their performance. Over 3 Olympics, so far, they are 18 - 0 in matches and 36 - 1 in sets won.
Also, watched the finals in women Heptathlon won by Jessica Ennis of GB. about half of the women wore bikini bottoms and half wore a different type bottom. It is their choice as to what they are most comfortable wearing. I wouldn't think about being critical of those that wore the bikini bottom.
LOL, have you watched any of the men's track and field competition. Nothing is more revealing than what some of the men wear. However, they choose to wear the suit as they are comfortable wearing it.