Have a read of this: https://lemmy.nz/comment/2326509

It's not about ruling out detaining people. It's that prison is a terrible answer to the problem. Some person thinks that crime is acceptable. Let's put them in a building full other other people who think that crime is acceptable, so the only people the associate with are criminals. It's a system that makes more criminals, and it will be seen as obviously a bad idea in 100 years.