Halloween is the one night when it is perfectly acceptable to dress however one wants. There are no fashion rules, no "omg she didn't's" its whatever you can buy/find/make goes. So naturally women who have to wear uniforms , have strict dress codes , or just couldn't justify dressing trashy during normal life, dress a little "slutty" as you put it.
And you said the point of Halloween is to have fun? right? So what if those girls ARE having fun? Who are you to criticize what they chose to wear?
Im assuming that you might be insecure of your own body, or come from a conservative household, or are just one of the crazy feminist.
In either case , just let the "sluts" be. They aren't stopping you from having your fun!!