AITA for ripping up my stepson's friends' concert tickets?

So I (28M) recently took in my stepson (14M), we'll call him "Greg". He was living with his abusive birth father, and even though his mother is no longer alive, I see him as my own. I'm more than happy to take on this responsibility, but naturally, it's taking a minute for him to come around. I know I should expect him to act out, seeing as how horrible his birth father was to him, but he did something very disrespectful today and I don't know if I overreacted.

I was going away for the weekend with my girlfriend. I'd dropped off my daughter (Greg's half sister) at her grandma's house so he could have the place to himself. I knew he was going to have his friends over, so I figured I'd establish some rules rather than setting him up to get in trouble. As I was leaving, I saw him come home with three of his pals (we'll call them Darko, Shane and Zoomer). I felt a huge wave of relief knowing his first instinct was to listen to me.

A few hours later, I received a call. I run a used car dealership, and the silent alarm had been triggered when I wasn't there. I floored it back, only to see Greg and his friends had stolen one of the cars off my lot for a joyride. The thing is, I'd let him take it for a spin earlier. He knew he wasn't supposed to drive it without me, but obviously that didn't stop him.

His reasoning was that it was the only way for them to make it in time for a giveaway for tickets to see their favorite band, Kid Elrick. Greg and I are both musicians (he's in a band with his friends), and nothing makes me happier than seeing him embrace his creative side. That's why it hurt so much when, without even thinking, I tore up all four tickets to the Kid Elrick concert

I know Greg's been through so much and I know how excited he was to see Kid Elrick. I wouldn't have let him go to the show no matter what, but it wasn't until I'd torn the tickets that I realized I was also punishing his friends. The last thing I want to do is make their high school experience any more difficult than it needs to be, but without consequences for his mistakes I fear he could end up going down a dark path

So Reddit, AITA?

Update: his dad died