It's just like employment discrimination. In an at-will-employment jurisdiction, my boss is free to arbitrarily fire me, but cannot specifically fire me for e.g. my race.
(How do you tell the difference? Well, as in this case, usually people leave some sort of evidence, and the legal system has a process for demanding evidence and evaluating it. It's not perfect, but it's certainly not unenforceable.)
You’ve got it backwards. Instances where they do leave evidence are the exception, not the norm. Which is why the anti discrimination laws are somewhat toothless and the government tries to look for statistical evidence of discrimination.
(How do you tell the difference? Well, as in this case, usually people leave some sort of evidence, and the legal system has a process for demanding evidence and evaluating it. It's not perfect, but it's certainly not unenforceable.)