Tell me why, dummies:

Mechanics are not screened. Caterers are not screened. Cabin cleaners are not screened. Baggage loaders and apron workers are not screened. They are subject to fingerprinting, a 10-year criminal background investigation and cross-checking against terror watch lists, but they receive only occasional, random on-the-job checks. Many employees say they have gone years without ever once being screened. —The future of airport security by Patrick Smith, salon.com