One could make the argument that multiculturalism is in the First amendment of the constitution, the separation of church and state falls under the umbrella term multiculturalism.
Promoting tolerance amongst the populace for your neighbors is only the tip of the iceberg as far as "multiculturalism" goes, and that's hardly a goal limited to multicultural policy goals.
Multicultural philosophy is more about how the government itself interacts with those populations. Forced Assimilation to a single "way of life" is the other end of the spectrum. Tolerance of diverse political opinion, as well being able to dress how you like, sexual identity, are also part of a multicultural nation's identity. There's no more reason to force immigrants to dress and act like "proper Americans" than there is to force young Americans to all dress and act the same. Who is this proper American that the immigrant must
And it wasn't really that long ago that everyone had to conform to a christian, church-going, straight, conservatively-dressed "normality", otherwise bad, bad things would happen to you (probably locked up in an asylum or beaten to death in the street).
This is where the meat multiculturalism really is. We take for granted being able to pretty much act as we please without the government crushing us for not conforming to a monoculture. I read once about an Alabama man put in a mental institution in the 1943 because he told a colleague he had sex with his wife 4-5 times a week. He was diagnosed as a dangerous sex-addict. 2 times a week was the max. "normal" times.