I'm not defending the conditions, I'm simply asking what provision allows sit-ins in a private business? What you think is unjust and unfair? What I think is unjust and unfair? Heck, I have a list of stuff I think is "unfair" a mile long. Let's do it! Fortunately I understood long ago that life isn't "fair". Just ask the poor citizens of Cuba.
Parenthetically I suspect we all have a group that we treat as second class citizens be it due to baldness, age, weight, looks and so on. We may not even know we do it, but after many decades of watching human nature the founding fathers had it right as usual; you can be equal under the law but flawed humans have to be allowed to be asses; no way to law your way out of that one.