Eh, I'm not a parent but I can't imagine it's always that simple.
When I used to let my niece go with me shopping, I could manage it. Some parents today manage it. My parents manage it.
Your kid doesn't have to a perfect, quiet, angel walking calmly next to you, but some of the behaviors I see from with kids in public is astounding to me the parents aren't doing something to get the kid to knock it off. Running around aimlessly while the parent is just chatting with someone, or looking at a product? You can't tell me that that parent can't stop what they're doing and give their kids a stern-voice "knock it off and stand by me!"
I saw a parent once she was shopping on one end of the aisle, I think trying to decide what kind of Hamburder Helper to get or some bullshit. Her kid was on the other end, closest to me, with one of those little-kid shopping carts the kid crashed the cart into a cardboard display thing, knocking it over, and its contents (unbreakable) spilled all over the floor. The parent heard the crash, looked up and over at the kid and in a calm, warm, voice asked the kid to come to her (In the same kind of warm voice you'd use to ask the kid to come to you because you want to clean them up or do their hair or something before going out of the house or some other calm, warm, thing) the kid ran down to her, cart rattling the whole way, he almost crashes into another shopper (no reaction from the mother) and the kid meets up with the mom. She say nothing to the kid. Moments later she walking towards me (and the spill) and she just simply walks by it. Says and does nothing. Didn't even tell anyone the spill happened. I finish up with my customer, then go and fix the display. The mother "boomerangs" back to my direction and walks past me, again saying nothing. No "sorry about that" or anything.
Oh, when she looked up the kid's car was clearly the one who caused the crash as his cart is still "crashed" into the display as right now his cart is supporting its weight. So when he moves it crashs and spills more.
So, tell me, did this happen because the woman is a rude idiot who can't parent or control her kid or because parenting isn't as simple as I think?
When I was a kid I once bit into a pkg of M&Ms I wanted. My parents were mad, being now forced to buy it, and when we got home my ass was made red from the whoppin' my dad gave me and then he ate the M&Ms. I never did it, or anything like it, again. Yeah, yeah, spaking kids these days is so evvvvvullllll. But you can't tell me this woman couldn't of been a little more stern to the kid who just made a mess and done something like make him pick it up or be the one to tell someone he spilled it. Nope, just a warm "come here" and walk off.