My pop-psychology spider sense tells me this is the important factor. People don't like to be made aware that their locks are actually useless. This unhappiness gets taken out on the locksmith, because he happens to be around. It's shooting the messenger, which is less than useless, but still very common. Conversely, putting on theatrics to open the lock makes people feel secure. A false sense of security, obviously, but one that results in better tips.
They aren't useless. They raise the bar from "able to open a door" to "able to pick a lock or willing to break the door down". That cuts out like 99.9% of people on the planet (and a vast swath of actual criminals). That's a good deal for twenty bucks, even if the bar being raised is almost comically low.
What people don't like is paying for someone else to do something which appears trivial enough that anyone could do it. That it's a skill which takes time and practice to hone until the process looks that trivial is irrelevant, it looked too easy to need to pay someone else to do it.