They do have to do testing, but honestly it is real easy to get past it. Plus, they don't have to do it all the time... people can change, for the good and the bad.
It comes down to this... Disney property isn't locked down and, even if they screen every person who comes in, people will still get by. That is how you get clean background checks on people who HAVE committed crimes... they just were never caught before. You have to take personal responsibility for your self.
Computers come to mind.
Every day someone figures out a way to outsmart the hackers and every day they come up with something new.
I agree, you cannot prevent EVERYTHING in life, but you can be diligent in being aware and protect yourself from what could possibly occur.
Unfortunately, many think it is everyone else's responsibility but their own.
It should also be noted that this thread title is also misleading, just like the original thread. They weren't "robbed", there was a "theft" of items - that were quite frankly left unattended.