Arabs, so far, appear to be up the major component of the Jihadist terror movement. In other words it is not far fetched to say "arab = (at the very LEAST) potential terrorist. What is there about this crystal clear concept that you do not understand?
Let's say there have been 100 illegal Arabs arrested for terrorism in the last year in America, out of, say, 100,000 such in the country. I mean real terrorism, not bureaucratic diddly like overstaying visas.
The thing I don't understand about your 'crystal clear concept' is how one-in-a-thousand is anything BUT far-fetched.