Do you guys think that I was unfair to my son regarding this long ago situation?

I was reminded of this incident recently so I thought I’d see what you thought of it.
When my son was about 11 he was out playing with his friend, Chad. Chad’s dad brought Chris home later and said he and Chad had broken out a car window with a rock from a sling shot. He then handed me this giant sling shot. He said, “Chris said it was his.”
I was like, “Chris? Where did you get this?”
Turns out Gramma (my mom) had bought it for him at Walmart. He had the money to buy it himself, but you had to be 18 to buy it and he was only 11. IT WAS A HUNTING GRADE FREAKING SLING SHOT!!! It was no toy! It was in the sporting goods department where they also sold rifles and shotguns (back then.)
Chris just had a feeling that I’d confiscate it if I saw it, so he hid it from me until he had a chance to try it out….
They were in a tree, about ¼ of a block from the car and shot the window out. They weren’t being malicious…I’m sure it shocked the hell of them when it shattered.
It made me sick to know there were kids from the neighborhood playing in the vicinity too. It could have been SO much worse.
Well, the guy’s car window need to be replaced, obviously. Chad’s dad and I agreed we split the cost, about $80 each. But then I turned to Chris and said, “That’s about how much you have saved up for a new bike.”
Chris hung his head and said, “Yeah.”
Well, Chad’s dad heard that and said, “No!..I…That’s a little harsh. I mean, Chad was the one who actually shot the window out.”
I said, “But it was Chris’ slingshot. Chris, when you own something, and you let someone else use it, you’re responsible for what they do with it.”

Anyway, Chad’s dad thought I was mean and it just wasn’t fair to Chris.

What do you guys think.

(Let me forestall what you’re going to say…yeah, I think my Mom was more responsible than anyone. But the fact that she BOUGHT the thing for him, without even wondering why he had to be 18 tells you a little something.
I did tell her about it. All she said was “Really?” Like, that’s interesting.
I said, “You know, there were kids around. Someone could have been really hurt.”
She said, “But they weren’t so it’s OK.”
I said, “Yeah, and Chris lost his bike money to pay for it.”
“Oh, that’s too bad! But when you do something you have to pay for it.”
It was like she had absolutely NOTHING to do with it. I honestly don’t think she ever made the connection either. Knowing my Mom, I wasn’t about to lay it on the line, either. She would have just freaked and tried to turn the responsibility somewhere else.)