Obviously, you can't use this strategy against U.S. citizens or legal residents because due process applies; we need to consider better ways to deal with security threats under due process, but this is an old argument (all the way back to the Alien and Sedition Acts).
I know there have been cases in which some of these front "charities" have been caught . Although not tied to Hezbollah, five leaders of the Holy Land Foundation were convicted of providing material support to Hamas in 2008. The FBI had been investigating them since 1994 (No Cash for Terror:Convictions Returned in Holy Land Case, 2008, para. 1,4,8)
I guess I have to reverse my comment in the first paragraph...it looks like the use of "charity" fronts is a long used tactic of terrorist facilitators.
No Cash for Terror:Convictions Returned in Holy Land Case. (2008, November 25). Federal Bureau of Investigation. Retrieved April 27, 2015 from http://www.fbi.gov/news/stories/2008/november/hlf112508
I also think that there is a normal human tendency to stick one's head in the sand and to ignore problems; this is evidenced by a strong strain of isolationism in American politics. People don't want to roll up their sleeves and get work done to avoid problems, but they will react once those problems get big enough to bite them on the posterior.
U.S. District Judge Royce C. Lamberth said the suicide truck bombing was carried out by the group Hezbollah with the approval and funding of Iran's senior government officials" (CNN, 2203, para. 2). This goes back to my and x's contention that Hezbollah is a front group for Iran.