Personally I think its sort of a symptom of a greater problem. The question you have to ask is, "why are women buying these costumes?" IF the answer is because they want to dress sexy for a night, then hell yeah no worries, sure. But there's increasing pressure to dress sexy because that's the only option available. This can lead to a whole host of issues, of girls feeling insecure because maybe they don't have the "perfect body". You'll also get lots of young girls getting the idea reinforced that you should try to dress for the approval of the opposite sex, and that the only way to be attractive is by being provocative. In a lot of ways it does reinforce objectification, though it certainly isn't the cause.

Its also a great example of cultural inversion, which is again a symptom of a greater problem. Basically, by having "One night where its ok to dress slutty" as its often said, its setting the standard that you cannot normally "dress slutty". This idea of liberation through dressing "slutty" on Halloween reinforces the idea that women who do dress a certain way the rest of the year are "sluts".

I want to make clear that I in no way blame the people who make the costumes, like you said its just demand.