Basically anything at all is easy to obtain in prison if you know the right people, despite the best efforts of the prison officers and police. Think of all the times you see photos/videos from inside clearly taken on mobile phones, the many reports of prisons having rampant active drug problems including prisoners coming out with habits they didn’t have before, all the articles recently about the issues with drones.
Bearing all that in mind, whether or not the tins are supposed to be kept in a secure area or the kitchen only or whatever I’m not surprised it’s relatively common for prisoners to have them and use them as weapons.