Anyone who is loud and proud of not tipping waiters and delivery boys and goes out of his way to get a kid fired has no soul and probably has other problems.
In a word, he is an A$$.
Are you assuming that the kid did nothing whatever to deserve his treatment. Do you think a rude waitress deserves a tip?
I had a waitress accidently spill an entire milkshake into my lap. Did she deserve a tip? Perhaps it was not her fault and perhaps she didn't deserve to be fired, but I certainly didn't feel well served.
Perhaps the pizza customer said, "wait just a minute", so she could get proper change. Perhaps the delivery kid said, "well hurry up. I have other deliveries to make."
There could have been many reasons not to tip somebody who behaves in an immature manner. Certainly leaving a rude note was immature.
Further, you seem to be assuming that the delivery kid had no background of other misbehavior. Most employers are not anxious to have to go through the hiring process. If all your employees are competent, you should be thankful. Hiring somebody new is fraught with risks.
If the employer was justified in firing the delivery kid, why are you blaming the customer for bringing such important information to light? Is it your opinion that NOBODY ever deserves to be fired?