I hate to out myself, but also, I have been through the opposite situation.
When I first found my dog, it was an unexpected event. I found her running about in the streets and no one ever claimed her. I wasn't planning on having a dog, but at the time, I could afford to care for her and had the time to play with her. It seemed better than putting her in a shelter. After an unexpected financial crisis (job loss; trouble finding a job; had to get multiple jobs to make-up for debt), I had to work 60-70 hours a week (not to mention commute times) at shitty jobs and could not afford a lot for her. I couldn't spend a lot of time with her either. I tried many options, but the best decision for her was to rehome her. She is now with a guy who has way more money and time than me. And he probably appreciates her more than I could have. Had I not initially picked her up, who knows where she would be (probably dead, as I lived by a busy road). I found a way to get her in a home and not put her in the shelter, and I am at peace with my decision.
I do miss my dog everyday. I do still have that feeling of "something is missing" and that she is still my girl. I wish I could get her back, especially now that I've recovered financially. But the decision to give her to someone who could care for her better was for her. Not me. I could have kept her and she wouldn't have gotten enough attention or potty breaks, etc.
But for those people that aren't, shaming them is pretty shitty. There is ALWAYS someone who had it/has it harder than you, and it's easy for you to gloss over their hardships and chalk it up to something else. This is a bigger issue than just the people.
People can be dumb and make mistakes, but we can only hope for the best and try to help direct them the right way. What we really need is to focus on spaying and neutering and maybe be a little more thorough with the adoption process. Maybe Craigslist should just take off the "Pets" section completely. I don't have good solutions.
As someone who has been on the other side of the coin, the shaming hurts for someone who could admit they were fucked and made the best decision for their loved one.