Yeah, all that nice pseudo-liberal babble about how we're gonna live in peace and mutual tolerance. It works fine if the cultures are at least a bit similar and/or tolerant. When you mix two or more cultures of which one is intolerant and refuse to live in peace with the others, the whole concept goes down the toilet.
Countries and nations are entities based on internal coherence, which is to large extent based on a set of values, traditions etc. accepted by most of the citizens. Who doesn't accept them is thus a potential danger for the country, which is why he must assimilate or leave.