Because they work in an industry that is primarily based on looks. Just like it's "okay" to stare at fashion models, it's "okay" to stare at waitresses at Hooters. Seeing as you don't work at Hooters (I'm guessing, ( ; ) then this doesn't apply here.

> In a post-apocolyptic world where you're the only person on earth, would you dress the same way you do now?

I'm not sure this hypothetical adds to the conversation. I also wouldn't dress the way I do now if the gender dynamics in Western culture were different. What's your point?

> Being "attractive" specifically refers to other peoples' attention and emotions towards you.

I would argue that being "attractive" is more a question of self-esteem than perception by others; see any interview with Megan Fox where she admits to not feeling attractive. It's not about how others feel about you, but about how you feel about yourself. How you feel about yourself is, partly, a product of the culture that contextualizes you, but that's more about the cultural aggregate than specific individuals (the hypothetical woman who is commenting about my great butt).

> I'm not saying that it's ok to leer at women, to verbally or physically harass them, etc. But it's incredibly ludicrous to me to think that someone dressed in a manner usually considered "provocative" probably did so without any intention, or that anyone responding to that mode of dress is automatically sexist.

Totally. I know you're not advocating horrible behavior, but sexism is not a flipped bit; it's a complex spectrum with multiple dimensions that is hard to quantify as "on" or "off". I tend to think that most people (me included) are sexist, and it's not categorically Bad and Evil, just something that I consider in my responses to certain situations and issues. It's okay to be sexist; it's not okay to make unwelcome remarks about a woman's appearance.

But you're asking the wrong question here: why is someone wearing a low-cut dress "provocative" at all? Provocative to whom? Why does how I dress necessarily have anything to do with anyone else? Every woman who wants to wear tight clothing wants to be looked at, otherwise they would wear burqas? It's incredibly ludicrous to me that you seem to be putting forth that supposition. It seems to lead to, "I'm sorry she was offended, but if she didn't want to be offended then she should've dressed differently." Which is a form of victim blaming: http://en.wikipedia.org/wiki/Victim_blaming

As to coming to work topless: no one is questioning whether or not being attracted to women is sexist. Of course not. But commenting about a female colleague's cleavage at work is sexist and demeaning. Be attracted all you want, fine. But don't cross the line.