Cannot understand

I cannot understand one thing - few days ago, on housing page on Facebook, group of Dutch guys students posted about looking for a housemate but they asked for "Dutch only”. In comments you could see a lot of hate from non-dutchies, accusations of being racists etc. What’s wrong with those ppl? If someone wants to live with only Dutch ppl is it a crime? I’m from Poland and I was extremely surprise when I saw that accusations - mostly from ppl, let’s say, "different cultures”. What do you think, is this a racism? Or people start loosing their minds?