Prison isn't close to a hotel. It isn't the prison itself that cost money, it comes with a lot of administration, guards etc etc and the more prisoners the more it's going to cost. They might have the "gang-culture" but most of these young "criminals" aren't actually in a gang. But when they sent to prison they are introduced to a lot of real gang members and most of the recruitments happens in prison.
Even if they have this "gang-culture" prison isn't the answer, it's going to make things worse which is proven.
They are aware of what they have done but they might not be aware of the consequences.
"They should be punished not having a chat with some old lady about if they have it rough at home"
Who said something about having a chat with some old lady? lul
There are other punishments than prison that are available.
Prison is the last option when used as punishment and should be treated as such. That's why we don't send these kids to prison because it's not the correct punishment.
Also, these kids most likely don't have any citizenship because they are most likely born in Sweden.
"Would be nice if their parents got the bills for all those cars so they'd beat the fuck out of the kids who did it"
Nice that you want adults to beat kids. I bet you are going to raise your kids this way