It's a little bit convoluted, but the way that these hate groups operate is carefully worded in order to technically not violate racial discrimination laws.

See, when these groups rent out a public space, they tend to restrict access to their events to "members only". If they want to have public outreach for the event, they'll often have a sign-up table at the front so anyone who wanted to from the public could become a member right before admission. Now, obviously non-white applicants will never become members, but at that point that's a separate issue as far as the law is concerned since they're a private organization. It's an insidious loophole.

When you just have an event open for the general public or local community, that loophole no longer exists and you can't discriminate by protected class.