Yep. Kids aren't stupid. They begin manipulating at 2 yrs old. Parents that don't get their kid under control early in life, normally have larger problems down the line.
For a screaming kid, you pull them aside and tell them to stop the crying or they are going to get spanked and will not be allowed to come to the grocery again (normally kids like going to the grocery). I never did any of that shit when I was a kid and neither did any of my siblings, so quit with the, "sometimes you can't do anything crap." Get your stupid kid under control!
+1
A child under two is problem; the only thing you can do is shop the other side of the store.
Those 'out of control' are really obnoxious and as the OP said doing nothing when your child's actions are discourteous to all the shoppers and employees in the store is not the proper behavior of the adult. The fault lies in the parents, not the kid.
That doesn't mean you have smack the kid but if necessary, the parent should stop shopping and take the kid outside. The kid will get the message soon enough. My day, my progressive parents. enlightened by Dr. Benjamin Spock's book, would have added extra chores which would have cut into my play time, But like
JackBurton I didn't do that neither did other kids.
There are tactics you can apply in a restaurant or Pizza place to counteract such irresponsible parents but in a grocery store it is difficult without being excessively rude. The fact is the parent, because of his or her failure to consider others, is being excessively rude. I less couth shopper might say do something like referring to a women parent as a female dog or a man as the male offspring of a female dog; the once getting the irresponsible parents attention say "Shut the 'expletive deleted' kid up!" While I won't say it; I would join in with a 'here, here' say 'my thoughts also'.