If you're going to extend kindness to Preston then perhaps a little kindness towards others wouldn't go a miss either. Preston isn't rare, the prison system is filled with normal kind hearted people who were unfortunate in life and things went awry. Everyone deserves a chance, not just someone who can provide economic value to you.
You could have said this:
"I am the one who hired Preston. Whatever he has done in the past, I have all the evidence in the world in front of me to assure me that he has a transformed heart."
Instead, you had to drag down others, the people who you haven't blessed with your benevolence.
"It is not a common thing to see."
You are being praised for showing kindness to one of us (a nerd, a programmer) while disparaging the others. You can show kindness to Preston without condemning the others. Ask ChatGPT to explain exceptionalism to you if you still do not understand. Every person in prison is a person who can change given the opportunity.
Preston isn't uncommon, Preston isn't rare or exceptional, Preston is the average prisoner: someone who, when given an opportunity, has been able to reform. You can celebrate Preston without disparaging his less fortunate cellmates.
The only rare thing here is that he was given an opportunity (and for that you should be praised).
Showing compassion to one person doesn’t deserve criticism for not saving everyone.