It's not that they should like it, but c'mon, nobody held a gun to their heads and forced them to be booth babes. They're the ones who chose to make their living primarily with their looks, combined with the ability to stand around and smile all day and act like they aren't bored.
$100 - $130 for 2 hours work? That's pretty good money. Being a booth babe isn't full-time work, but for the hours they work, they make more than most software engineers. I bet strippers don't make that much, either. If the job is so onerous, they are free to quit and find some other job that uses whatever other skills they possess.
WRT the standing in heels thing they talk about, they just aren't spending enough. My wife is a hair stylist, and she usually wears heels. She says you have to really spend to get comfortable shoes. I can tell you that her shoes are pricey, as are the tools of her trade. I view the shoes as a tool of the trade. You wouldn't believe what a good shears costs. The shoes are cheap compared to that :p
As to whether having booth babes is sexist or not, I'd say that it's not. Sexual? Sure. But that's not the same as sexist. Sexist is denying someone a promotion, raise, or job because of gender. Hiring a couple hot women to wear cocktail dresses (or handsome men to dress like Chippendale dancers) and stand around holding your product at a trade show is sexual. There was nothing sexist about the Asus tweet, or even untrue. The model does, in fact, have a nice ass. But there is no requirement (as far as we know, anyway) that their customers must have nice asses to be allowed to buy the product, or that their employees must have nice asses to get raises or promotions. Such a requirement would be sexist.
Was that tweet stupid? Yeah. But what is Twitter, if not a service to assist in proving to the world that you're dumb? Was it sexual? Yeah. Was it sexist? No.