I'm of two minds about treatment orders, the way they are where i live. Can be good or bad.
The injection itself you can receive by choice or medical recommendation, not just from order, and i know some people who prefer it, or are helped best by it. However, it can be difficult to receive if you don't qualify for case management where i live, and i do not agree with that.
The shot goes in your hind end because it needs to go into muscle tissue. Your butt muscle is the biggest on your body, and thats why its the best choice. I don't think they put it in the rear end for added humiliation. I've had other, medical (not psychiatric) shots that I've had to receive this way.