Never understood that either. If someone was wrong and bad, and now they're trying to do right and good, we need to celebrate that. Not just because that's awesome in itself, but also to give the opportunity and incentives for others in the future to do better.
If everyone is always bad regardless if they're trying to change, what incentives would they have from changing at all? It doesn't make any sense.
The other part of the equation is not letting bad people get away with doing bad stuff if they do good stuff after that. The return on doing bad stuff, then good stuff has to be greater than the return on only doing bad stuff, but less than the return on only doing good stuff. It should increase over time the more you don't do bad stuff again.
I agree with the sentiment (people changing their minds), but the flipside to that is people pleasing. Someone who capitulates under even the slightest pressure is not much better than the person who is set in their ways.
The trouble there, of course, is that the motivation for changing (or not changing) one's mind is not always clear, and it's easy to score points from spinning it one way or another.
The incentive is less about morals and very much about self-preservation.
With online mobs, when the target shows any sort of regret there is blood in the water and the sharks feast. It sometimes turns into a very public form of struggle session for the person under scrutiny. Besides avoiding the faux pas in the first place, one well-tested mitigation is to be absolutely unapologetic and wait for the storm to blow over.