They are meant as a deterrent, however the deterrent does not stop people who feel that the threat/risk of being caught is either less than the chance of success, or those who simply don't care because they never bothered to work out an exit strategy.
Guess what. The guys who never bothered learning how to land an aircraft never planned on landing it. Many of these folks plan on dying in their attempt. The minute they are confronted with equal force, they off themselves, and we are supposed to disarm? Please. Don't talk about being clever. I think you have no idea about the definition of the word.