They're humans

It's what they do.

Today's ethical problem is a customary guide to human rights. Human beings have some pretty odd and iffy customs which might result in

1. The right to torture people.
2. The right to own slaves.
3. The right to infanticide.
4. The right to kill and consume the elderly.

If human rights was based on customs, then this is the sort of ethically dubious list you might end up with. And all these things have been and are still being done. The first three are alive and kicking, and I'm sure there's still some bunch of cannibals regularly serving mutton dressed as lamb.

Of course, the custom-based rights above are all contrary to what we regard as decent behaviour in the West.

The discussion in the book notes problems in formulating the UN Declaration on Human Rights because what we might regard as universal such as freedom of religion (aka the right to be wrong) and women's rights (although it should be unnecessary to have specific rights for half of humanity) were regarded as no such thing by such socially advanced societies such as Saudi Arabia.