It was a unique opportunity.
From what I have read he usually flew direct to the UK.
The few times he had been in the US, he was in Washington to lobby. It looks really bad to arrest someone who comes to the government to legally lobby for a cause.
Anyways, the feds took advantage of opportunity.
Now what I would like to know is this:
How were the feds able to know a guy named David was going to fly into Dallas yet when 19 of the world's known terrorists with names like abuqua all boarded planes within minutes of each other no one noticed.
-Sean