Because everyone has different opinions, and some people are worse at timing, and at becoming better than others are.
Because not everyone can be fair, every time, and they shouldn't be asked to. Because the human heart can only bear so much.
See, redemption isn't just about the person seeking it, it's about those they have victimized, too. The person changing can truly do so, but that doesn't mean a former victim will ever be able to see it, and it doesn't mean they ever should be.
The person changing should be given a chance, but if someone can't deal with them, they should not be forced to, as long a they don't stop others from doing so.