Because you can get killed! in Vietnam. This is the same as showing off your body could get you in trouble also, such as rape and killed because they look at you as a hooker.

http://tuoitre.vn/Chinh-tri-xa-hoi/Phap-luat/568607/1-phu-nu-bi-giet-chet-do-deo-nhieu-vang.html