Its not the relatively small number of immigrants themselves that worry me. As you say many of them are searching for a better life and there are probably also a minority of radicals that slip through.
Its their descendants and lots of them that are the problem. Their parents may indeed be moderate but their kids, grandkids will go to the mosque, read the book and choose their own path and may turn out very different to their parents in this century or the next.
Like an infected colonist from the movie Aliens its the seed of evil they carry within in that diaboloical book and its teachings. Ready to sprout any century in the future.
Better the aliens are kept on their planet rather than infect new clean worlds.
