In all fairness, you have no idea what sort of stunts that child has been pulling all day, or how patient the parent might have been up until the point where they lost it. I say this from experience. I love my oldest son more than life itself, but he can be a very challenging child. At his best he's sunny and helpful and enthusiastic, but at his worst...watch out. Our first
Disney cruise, when he was six (almost seven), he spent the first two days in such a snit that we couldn't do anything with him. We tried all the "good parent" tricks you can think of. We tried to jolly him out of it, we tried ignoring his behavior, we tried talking seriously to him, reasoning with him, etc. Didn't matter. He was in a bad mood and determined to make everyone else just as miserable as he was. Finally on the second night, he flat-out refused to get dressed to go see "Aladdin." My DH had had enough, and frankly so had I. He said "You can't make me go." DH said "Actually, yes I can" and dressed him like he was a two-year-old. DH then proceeded to take him by the hand and literally drag him to the elevators, then dragged him all the way down the ship to the Walt Disney Theater. He didn't yank him or hurt him, he just kept walking straight ahead with a tight grip on our son's hand. It was actually pretty funny. My son got tired of being dragged while lying like a rag doll and instead stood up and literally dug in his heels. So DH dragged him that way, with his heels skating along the marble floors. And when we got to the theater DH picked him up and sat his bottom in a seat and said "You will sit here and watch the show quietly OR ELSE." Or else what, I don't even know, but my boys generally don't try to find out. And after that we didn't have a single issue with him for the rest of the trip.
My point is that I'll bet there are a lot of people who watched this spectacle and were appalled. They probably thought, "Is that really necessary? You're on vacation! Just have fun, talk to the kid, why drag him and make him do something he doesn't want to do? And who drags their kid around like that, anyway?" But we had had 48 hours of this child thinking he was running the show and being a pill just for the sake of being a pill, and we were done. And you know what? That was the end of it for him, he was a doll after that for the rest of the trip.
I also would like to say that as a parent, sometimes I feel like you can't win. If you don't reprimand your children, you're a lazy parent who's raising entitled brats who are obnoxious and don't behave. If you do discipline them in public, you're an awful parent who is mean and can't even let them enjoy this wonderful vacation.