...Except that in the US, we don't have nationwide bans on what is acceptable to wear in public (save for nudity/lewdness, as well as things like ski masks in banks, for obvious reasons). You enjoy religious liberty because the United States favors Christians despite the fact that we are not a Christian nation -- Shariah-run nations of the middle east require certain levels of conformity based on the presiding religion, but we have very loose definition of what is immodest dress and a very broad culture that encompasses all kinds. In the US, Christians are allowed to display crosses, members of the clergy are allowed to go out in the streets in their religious dress, Jews can wear yarmulkes, grow sidelocks -- but hijab is responded to with fear and misunderstanding, and somehow it's okay to ban that, but can you imagine the outrage if it were decided that Christians were unable to wear anything associated with their religion? How do you make it okay for one but not the other except to say that one is more normalized and familiar to the population at large?
Thing about the United States is that we have built our country on immigration -- we don't have an assimilated culture, and I might venture to say we never have. I don't know what period of "similar attitude" you're referring to, because there have been culture clashes with every new wave of immigrants to enter the country -- even European immigrants way back all the way to Hispanics today.
I agree with you that immigrants have a duty to assimilate and participate in their new country to become active, engaged, contributing citizens, but the place they came from is something that helped shape them -- it's not just going to go away. It's inevitable that they will expose their new country to non-native traditions and customs, and I think it can be a beautiful thing. You can still have things that are "uniquely American" much as there are things that are "uniquely French", but the world is not a static place -- it is dynamic, constantly shifting and evolving, and so too must our definitions. First-generation immigrants are often the least integrated, besides -- it's not until they have children who grow up in the US and then have children of their own that you really start to get accustomed, fully adjusted citizens.