I hope they can either get the kid quiet quickly, or remove him from the store/restaurant/wherever. My son rarely threw tantrums (I was lucky!), but the one time I can remember him doing it, I took him outside immediately, no one else should have to listen to that. Found out later he would have those moments at school sometimes and had to talk to him about it. Turns out he was so over stimulated he would break down, so letting him go out in the hall was the best thing for everyone. I saw one kid go nuts at checkout right when his mom was paying, threw himself on the floor and everything (because he didn't get a candy bar). The mom told him to get up and he wouldn't, so she grabbed his arms and tried to make him stand, he just flopped himself around and screamed louder. So she dragged him out of the store, to lots and lots of applause. I'm sure she was frustrated and embarrassed, but instead of people gripping about the bratty kid, they were talking about how they appreciated how she handled it.
My brother's kids were horrible, even when they were 11 and 9 :( One time they whined so much and so loud when we went out to breakfast, *I* had to leave the restaurant because I couldn't handle it. SIL was doing nothing about it and my brother had "not been allowed" to do anything for so long, he just sat there too. It wasn't until my niece was around 12 that he'd had enough and finally started "getting on her" when she behaved like that.