Because every time a minority group tries to claim space for themselves, people in the majority group claim it's the same thing as when the majority excludes the minority.

Because at its core, as a matter of principle, making a PoC-only space because the discussion would be 'disrupted' by the mere presence of white people is bad, and in the same way that if a museum in Zimbabwe which had a section on Mugabe's racial policies had an exclusionary area for whites-only to discuss, because PoC might 'disrupt' the freedom of the conversation with their presence. It doesn't mean it's as bad as legal segregation. It doesn't mean it's the same as turning away minorities from a business as a whole. It doesn't mean it's as bad as 99% of racism in modern society. But it is still springing from the same fundamentally flawed principle that racial exclusion is acceptable.

It's happening in this very thread for goodness sakes. People claiming racism against white people, completely ignoring what makes racism so damaging in the first place. As if every instance of discrimination is the same because they look kinda similar on their face.

It is racism against white people, by definition, unless you're using 'power+prejudice' definition, which would render all sorts of racism suddenly 'not racism'. Hate crimes by black folk against Muslims in the environment after 9/11 would no longer be racism.

Just because the racism is not as bad (and, distinctly, it is obviously and apparently not as bad as colonialism, or mass segregation by law, or the intermittent exclusion of races in individual businesses, or modern societal-level cultural prejudices) does not mean it is not bad. Just because it is a minor incident of racial exclusion from a public venue with good intent does not mean it is 'not bad', it just means you probably don't need an international embargo against Germany until they fix it. It's still bad.

If someone, say, in this thread, commits well-meaning microaggressions against people of mixed-race without malice and with only the intent of buoying minorities not of mixed-race, not tearing down mixed-race individuals, that's still bad, and on the same principle that all racism is bad. That doesn't mean that they're as bad as people who call mixed-race people slurs - it doesn't even mean they're even close. But it does mean that it's still bad. It still fundamentally springs from a principle which should be examined with a critical eye in order to root it out for all incidents going forward.