Because “discrimination” is just, in other words, preference.
The idea that everyone in the world is the same, or should be treated AS IF they are the same, has no basis in reality or reason.
Equality is one of those things that sounds good at first, but when you think about it, it’s actually both anti-human and impossible.
What, so it should be illegal to “discriminate” in favour of one’s own spouse or partner, yes? Because everyone else has an “equal right” to “equal opportunity” to our attention and resources?
Sexual preference should be criminalized, because, by definition, it treats other more favourably, or less, on the ground of sex?
Every single human action without exception involves discrimination, involves preferring one thing or value over another.
What would be an example of a human action that does not involve preference, and therefore discrimination?
There is no way to defend the idea that discrimination is bad, without instantly descending into self-contradiction or absurdity.
What, no-one is to prefer their own family on the ground of their genetic closeness?
Equalitarianism is an anti-human doctrine, and has been responsible for the biggest mass murders in history, and for all the conflict that you are inadvertently promoting.
People have a HUMAN RIGHT to be FREE to associate with, OR DISSOCIATE FROM, anyone, for any reason.
The US Constitution was intended to protect freedom, not a doctrine that no-one is allowed to decide for themselves whom to associate with, which is indeed the OPPOSITE of the concept of freedom.