Do nightclubs operate on a sexist premise?

Business in modern society is regulated by laws that prevent one demographic from being taken advantage of over another. Anti-discrimination laws have been in place since the human rights movements and have since encompassed those with mental and physical disabilities. Businesses, under this pretext are not allowed to charge one demographic more knowing that they will/must pay for it.

Why then, regardless of "business intent" do clubs insist on "ladies free before..." and "ladies free on guest-list, guys 20$" without coming face to face with the most evident of anti-discriminatory laws?

Granted, some companies have the ability to circumvent these laws. Insurance companies are able to impose an increased premium on certain demographics based on probabilities and statistics. This is nevertheless, a regulated discrimination. One may argue that such a regulated discrimination based on an umbrella policy is unfair, but it is logical nonetheless.

In a modern society that has just seen the rise of 3rd wave feminism and near perfect equality in the eyes of the law, why does this persist? And why do the feminist groups not target this obvious discrimination?

All opinions are welcome!