Why do so many people that consider the sexual revolution/liberation to be a great thing, have a problem with pick-up artists?
because puas aren't promoting sexual liberation, they're promoting a social environment where it's encouraged to be predatory and deceptive. and hey, using cheap tricks like negging might screw with someone enough to get a dude laid, but it won't help them find long-term joy and satisfaction with women (sexually or otherwise). tl;dr there's nothing revolutionary about it.