The blame for this kind of behavior is not based only on the child but much more so on their parents and upbringing. Although you can f ind slutty girl syndrome everywhere as everyone is different there are some factors that encourage this behavior and they are as follows:
1) family, little or no parental supervision and proper parenting lead children (then later adults) to follow peers or the mainstream for their behavioral guidelines, If you think back to the 1800s america was obviously different, women's rights aside, you found very few slutty women (unless in a brothel) but a lot more refined ones that knew modesty since they learned what it meant to be a woman from birth. If you have no one to instill moral values you will follow what is around you.
2) society, the media portrays women in a lot of movies, music, and commercials in negative ways, if society enforces those sterotypes its even worse (like in the US). For example referring to them as bitches, hoes, etc... Look at the shows just showing women drunk and exposing themselves, or trying to get men with money alone! while men are referred to as... players. Very rarely do movies portray a woman with shining examples of morals or modesty in young adult cultures so they are brought up to think thats how the world is.
3) comparing each other is what women do, we naturally look at other women a lot of times and wish we had some feature or another and many times dress to impress or out do and appear better then others. Whether its nature or nurture i am not sure but many species of animal follow the same behavior to seem more appealing for future family building. So its kind of normal for women to size each other up and try to outdo each other, the same is true for men in another sense, however... sitting on your boyfriend's lap is going to far.
Anyway i know what you mean, but not all women are like this, what you dont see is that a lot of women who dont act that way are actually not at the party you were at, and maybe sharing a romantic moment with their SO's, taking time with friends, etc... If you walk down the street you would probably see very few of the women you speak of until maybe night time so you have to think of the venue as well.
