If people have heard you correct him for being on the counter, they may think they are helping. I still wouldn't like it. I would just interrupt and say "don't yell, it scares him" or "don't yell at him, I want him to like new people."
In my case, "Don't confuse him, he IS allowed up there!" My cats are allowed on the table and counter, just not by the stove where I cook, so I would really not allow anyone to confuse them when they know where they are allowed and not. But as one of my cats greets people by standing on the island when people come in the door, they generally know that it is allowed in my house.
If anyone persisted in scaring or yelling at my animals, I would remove the animal from the situation or ask the person to stop or leave. It is my job to protect them. That said, sometimes my pets are happier up in the bedroom where they can relax and not worry about people in the house.
(you didn't ask, but if you do want to train your guy off the counters- get him a high place where he is allowed to be (a cat tree or table that he is allowed to be on) and then try a boobytrap on the counters where you don't want him. Empty soda cans with pennies in them on top of a sheet of tinfoil works great. When they jump up, they disturb the foil and send the popcans flying. It is loud but won't hurt them if it lands on them. Works best if they are jumping up, less if they are able to step up from a stool or chair.)