That "don't snitch" thing holds true in most prison cultures, the same as it does in many military, and business settings.
You're new to a surrounding situation about which you know very little, and your peers, be they inmates, soldiers, or executives, could be engaged in illegal matters which are connected to larger, more organized criminal ventures.
You "rat" on someone you see doing wrong, but the people you just "ratted" to are involved in the larger criminal endeavor, so the people you "ratted" on are the first to know that you did so...
You're new to a surrounding situation about which you know very little, and your peers, be they inmates, soldiers, or executives, could be engaged in illegal matters which are connected to larger, more organized criminal ventures.
You "rat" on someone you see doing wrong, but the people you just "ratted" to are involved in the larger criminal endeavor, so the people you "ratted" on are the first to know that you did so...
Prison is not the same as your community.