It doesn't matter if your bike is actually fixed, as long as the shop can make you believe it is fixed and get it out the door. I'm told by someone who worked in the medical field that it's done that way in hospitals. Basically, if no one can pin a lawsuit on you you've done a fine job.
People who fear punishment more than they fear doing wrong only worry about getting caught, not about doing things right because they prefer to do things right. If someone like that performs a mechanical or medical service for you, you're both just waiting for your luck to run out.
While it's true that most customers won't know the difference, you can never be sure which ones they are. And a chronic careless approach eventually gets someone hurt. If you don't care if other people get hurt, just remember that to everyone else YOU are other people.
Maybe most people do shabby work and only look out for themselves. But why go out of your way to deserve to be treated that way?