Bigotry.
You are assuming Muslims think like we do. They don't.
Those that are peaceful are like anti-Nazis in Germany during WWII - few, far between, and dead the second the Nazis find them.
Sorry, but I look daily at what's going on in the Muslim world, in their own lands, and I see a people by and large from their leadership to the man in the street that support sharia and believe violently imposing it on everyone on the planet.
A reason why US is less vulnerable than Europe is Muslims are part of our fabric: soldiers, doctors etc. Bill Bratton, New York City police commissioner, just revealed this morning that he has 1,000 Muslim officers.
We are not Europe nor are we part of the Muslim world.
As for the poll you cited, it was produced by Center for Security Policy, run by Frank Gaffney, a notorious Muslim-under-every-bed fearmonger.
CSP’s survey was a non-probability based, opt-in online survey, administered by the conservative group, the Polling Company/Woman Trend, a small Washington-based agency that has collaborated with CSP on other occasions to produce surveys about Islam and Muslims. (We learned this after reaching out to the Polling Company to get more details about their methodology, which wasn’t released to the public when Gaffney began promoting the survey’s findings.)
According to the body that sets ethical standards for polling, the American Association for Public Opinion Research (AAPOR), opt-in surveys cannot be considered representative of the intended population, in this case Muslims. The AAPOR says that in these cases, “the pollster has no idea who is responding to the question” and that these kind of “polls do not have such a ‘grounded statistical tie’ to the population.”
http://bridge.georgetown.edu/new-poll-on-american-muslims-is-grounded-in-bias-riddled-with-flaws/In other words, the poll saying half of American Muslims support sharia law is bogus.