The problem is it is not an all or nothing situation most of the time. Usually you get paid part of it and they have some convincing explanation of why they can't pay the rest. Suppose they paid all your wages but not your travel expenses - pending verification of some receipt or something. They paid all your regular time, but not your overtime charges, claiming that payment had to be approved by someone higher-up. I've had both of these happen to me (both were legitimate situations and I did eventually get paid hassle-free) - but how do you know it is legitimate until after you are paid? You can stop working until you get paid entirely, but if you have bills due, it can be better to get some money than none at all. There is also the whole issue of the relationship - if it is just a legitimate hiccup you might not want to sour a good long-term opportunity, so you make a judgement call - he's a good guy and would never screw me - and next thing you know you next months check is missing too and you are down thousands of dollars.

It is always easy in hindsight to say it should never have happened but that does not make it anyone's fault other than the people who agreed to pay them and didn't.