Their complaint is that they want to integrate entirely and can not. They do not want to be different, they want to be the same. They want their kids to be treated the same. And the claim is, regardless of how well you integrate, how well you speak you will not be fully integrated.
FWIW I will be moving to Japan next year. I don't care if the Japanese 'accept' me. I don't expect to be treated the same, I know that they're somewhat xenophobic, and some of that might be for good reason. I fully accept that I will be a guest in their country. My goal is to do my best to minimise inconvenience to others and prioritise their cultural norms over mine.
I just highlighted some facts and compared it to other environments: many japanese do not give a hell about foreigners or interacting with them to the point they are not even interested in learning english and that is ok.
Well, yes, if you weren't born in Japan or born to Japanese parents, you will never be Japanese. And isn't that fine? I don't understand why somebody who has immigrated to a foreign country must be accepted like a native. Why can't one just peacefully integrate the best they can and accept their differences?