This is a classic refrain of apologists for bigotry. I belong to a country club that officially excluded blacks up until the 60's, and did so unofficially for years after. The policy changed under pressure from the community, which included bad publicity, public embarrassment, and disdain directed at individual club members from people on the outside. Public scorn for the organization prevailed over the objections of assholes who said, "it's a private organization! Go make your own country club!".
Eventually, Augusta may repent of gender discrimination due to similar pressure. And it still won't make the front page of HN because it has nothing to do with computers.
What is, however, equally legal, is to be outraged about it and to make noise about it. They can hold their event, I can yell at them for doing so. Both legal, so I’m not exactly sure what your point is.
And your broader points are just plain wrong: doing bad things in private doesn't make it OK. Doing things that are legal doesn't make them moral. Getting "up in arms about stuff" that doesn't directly affect you doesn't mean your opinions shouldn't be voiced.
People get up in arms about stuff like this because they're offended that people would so boldly exclude someone, but the key here is that it's a private event. Don't like it? Don't support them. Or, create your own private event with any rules you see fit.
As a nonreligious white male, I'm also excluded from minority scholarships, many religious private colleges, jobs with many religious organizations, being a waiter at Hooters, and plenty of other social groups, simply because of my personal beliefs or race or gender.
Women also can't join the Augusta National Golf Club (home of the Masters) for similar reasons as this, but I don't see that on the frontpage of HN.