I understand why people do it. It's social protocol, to show they're not upset. Or because they don't want to be the bad guy and spoil the moment. Or sometimes even because they don't feel it's the child's actions are that big a deal.
But here's the thing. It's not actually okay. Parents have to take advantage of the low-key opportunities to prevent the bigger outbursts later. It's not possible to teach during a tantrum.
By pushing the "It's okay" (and believe me, it's interesting how many people will want to argue with parents about that), the person is actually undermining the parent, distracting the child and prolonging the entire event. I'm not sure how it works with neurotypical children, but with mine, if I can't follow up within that crucial 3-5 second window, my opportunity is lost.
I've had other people tell me that I'm being mean and too strict when I've insisted on following through. But I stick to it, because their social approval isn't worth risking my kids' future on. They need consistently applied standards, or else it gets too confusing. And I don't owe the public an explanation or justification in the moment.