Americans have a lack of patience with immigrants and native-born who fail to assimilate into the melting pot. Some parts of our culture profess a love of diversity (a patronizing term, as it condescends to include those diverse elements that aren't part of our mainstream) or multi-culturalism (ditto). But that love generally deals with academics (read subculture Western and main culture non-Western literature) or tolerance (another patronizing term: don't beat the crap out of people who dress or sound different).
Non-assimilated cultures within the U.S. speak their own language and may know little or no English (or Spanish, for that matter), maintain extensive ties with often close family in their home country, live in cultural enclaves, send lots of money back out of the U.S., don't register to vote or vote if they're registered, and generally live as if they never left home. This pisses Americans off.
Strangely, though, we have a perfectly non-assimilated culture that's been around for hundreds of years in our midst. Fundamentalist Christians who hold God and their religious ideas above the law. They may be part of families that lived here from the Mayflower or earlier, but they won't recognize the supremecy of democracy in the material world.
Muslims have been generally easy to spot and thus to criticize for failure to accept our culture; fundamentalists bent on undermining our bill of rights are not. Some of them are even Cabinet-level officials in the government now.
Of course, what's ironic about this, is that every successive immigrant wave was "easy to spot" at one time: the Germans, the Irish, the Chinese, the Jews (of which I am one - most of my family arrived in the early 1900s from Lithuania, Danzig (Gdansk), and Russia). Each successive wave was seen as non-assimilationist, but they ultimately succumbed.
The fact is, though, that moderate observers of the Islamic faith may never assimilate in that way into our culture. The religion demands a lot of observance, and many liberal practitioners still devote more of their wardrobe and daily life to Islam's duties than the most fervent in most other faiths.
That observance is often attributed to extremism, when, in fact, it is cultural hygiene. I don't want to dispute whether the Islamic God is my God or someone else's God and whether he, she, it, them, or the platonic ideal of good actually historically or religiously requires the observance the Muslims engage in.
But it's clear that this observance, through community reinforcement, and, in its native incarnation, extreme penalties, has preserved the practice of faith over centuries with little let-up in its intensity of daily routine. Jews don't have it easy, but the so-called orthodox (who really practice a kind of 10th to 18th century version of mainstream Judaism) have daily prayers and other rituals that consume what I would argue is a less obstrusive, less demanding schedule. And there are millions of observant conservative Jews whose daily religious practice is minimal or non-existent, but who live in what they would consider a religious manner.
This has partly to do with the intermediation of religious life in different faiths. If you're a Catholic, you're responsible for your soul, but there are plenty of figures that can talk to God on your behalf: intercede or plead for you. Saints, priests, the Pope, Mary, and so on. In Islam and in Judaism, and in many ecstatic and Christian faiths, there's no veil between you and God. The rabbi or mullah doesn't speak to God on behalf of the community: he or she interprets God's words for the community to make their own decisions about.
Of course, the community may then decide you made the wrong decision. (This is why I like Judaism and Friends Meetings, both.)
When Muslims are singled out, it's another instance of cultural hegemony and imperialism. The fact that they have their dress after decades of living here, and that they persist in practicing their faith - well, it just annoys folks whose ancestors gave that up 50 or 100 years ago.