Part of the reason may be the implicit lessons we receive from a very young age. We must always be cautious of people who try to take advantage of us. It is a valuable less, because it is true, yet we seem to take it the wrong way. Rather than dealing with the criminal, the people who would defraud us, we treat everyone as criminal.
[0] There is a particular group of peoples in my country who are often portrayed as being lazy or exploiting the system. In many, if not most, cases this prejudice is just plain wrong. Since many of those people live outside of the system, they perform types of work that many of us would not even consider: hunting for their own food (and everything that involves), cutting their own fuel wood, hauling their own garbage to the dump, and much more. None of that work involves a paycheck, so it doesn't count. That attitude is just plain wrong. (Not to mention the other problems with prejudices, such as it discounting those who do function within the rest of western society.)
The biggest roadblock to improving the standard of living of most people in America, is this suspicious impulse folks have of suspecting somebody might 'get away' with something. It's time to recognize this unworthy impulse and discard it.