Who would do this to a dog?

This is Sloan's story.
Sloan is a puppy - a very young puppy. I don't know how to tell how young, because I'm not very familiar with puppies, but she's a baby. Her teeth are just finishing coming in. She is tiny. This is what she looked like when we first brought her home - scared and sad. She had only just warmed up.
It was a cold, cold morning in Denver. The sun was trying to warm up the streets, but the wind was blowing hard, sending gusts of snow flying across the ground. M. was home with QQ while I went for a quick morning run. Just as I got onto the path around Sloan's lake (yes, we named her after the lake), a man and a woman (unrelated) stopped me and asked if I could try to catch two little dogs which were runnning helter skelter on the path. The man explained that he (driving by) and the woman (walking on the path) had seen someone pull over next to the lake, open the car door, push the two dogs out, and take off.
The man had driven back to the maintenance office for the park and told them there were two strays. They had said they'd try to call the dog catchers - but the dogs were running fast and scared, and the man drove back to try to catch them himself. No luck.
I pulled off my headphones, bent down, and tried to coax the dogs to come near. They were very skittish. The adult dog - the size of a terrier and white - was extremely scared and did not want to be caught, snapping and growling every time someone came near. I focused on the smaller one. It was very fast and very nervous, and I couldn't even tell what it looked like, what with the wind and blowing snow. At first, I thought it was an adult miniature, but as I chased it around and it began to slow down, I could see it was just a puppy.
After that, I was determined to get hold of the puppy at least. It was so tiny, in that cold it would not have lived long. The older dog seemed like it would bite me if I even got near it. Finally, I got down on my knees in the snow, and the little one let me lay a hand on it. I scooped it up.
I took off my parka and wrapped the puppy in it, and walked back to the park offices. The man had told me that they were ready to take it to the shelter. When I got there, all the park employees were terribly nice. They all crowded into the office to take a look at the little dog. Each of them (all men) discussed whether they could take in a puppy. None could, though they were all regretful. The head guy told me that his regulations were to send it to the city pound. I asked about Maxfund, a no-kill shelter with medical benefits that we have volunteered for in the past. He said that would be much better - he knew of the shelter, but the city isn't affiliated, and he's a city employee. I said I would take the puppy to Maxfund if he'd lend me a phone to call my husband. He seemed very glad that I offered.
I walked back toward home with the puppy wrapped in my coat, shaking violently. It very quickly began to snuggle, shoving it's head up under my chin and licking my nose from time to time. M. met me halfway with the warm car. At home, we could tell how young she really was. She was hungry, and ate and drank heartily, but seemed very healthy and not overly thin. No infection of the eyes or nose, no jaundice. A good-looking puppy.
As she warmed up and began to relax, her little personality started coming out. She is sweet and extremely effectionate, loves to cuddle, and follows on my heels everywhere I go. She is playful and full of energy, rolling and tossing toys around, skipping and climbing on things. She even seems to be half housetrained - she pees outside like a good girl, although she did poop on the floor once when I took QQ to Target to get food and a collar for her. I'm kind of amazed she's even partly housetrained, as young as she is.
As I write this, she is curled in my lap with her head on my arm. She hardly weighs anything.
At the moment, M. and I are 50/50 as to whether to keep her or take her to the volunteer shelter. She would be snapped up, I know. Good-looking, healthy strays like this who haven't lived on the streets, particularly puppies, are in high demand. On the other hand, we were sort of half-looking for another dog to replace the one M. lost last summer. Still, this was nothing like what we had in mind. We never wanted a mini-dog. Plus, with a toddler in tow, it would be a serious challenge for me to train and look after a puppy.
We shall see.
But...here's my question: who does this to a dog?? Who pushes a tiny puppy out of a car into the cold. Who throws away a perfectly good dog? I still feel wounded for the dog that wouldn't be caught. It's a tough time of year for a small dog on the streets. I hope someone was able to catch it eventually...even if it was the dog catchers. Cruelty to animals, that kind of heartlessness, always cuts me to the bone.