Hey, religious person here, let my give you my perspective.
I think that smoking weed (or cigarettes for that matter) is wrong for me to do. However, I don't think that it's my responsibility to punish people that do things that I personally don't do unless it's going to hurt someone else or cause them immediate harm. It's not really my business if someone smokes or drinks on their own time as long as they arent going off and doing something like driving or of they decide to make it my buisness like doing so on campus.
Now, as for what makes snitching on people that aren't affecting others a bad move that puts you (and therefore Christianity) in a bad light, we live in a society that doesn't really offer many chances for forgiveness for low-level drug offenses. And having the capacity to forgive is pretty important to Christianity.
If you truely cared about the people who you see sinning you wouldn't turn them into the authorities as a first step (without a victim or if they're acting in a manner that makes them a danger to themselves and others. If they were, say smoking weed in front of kids or smoking and driving that's a whole other issue and you would be nta for reporting that.), you would talk to them about how concerned you are about them, and then respect their free will if they don't want to talk to you about it. Respect is key, always. And letting someone know you care is better than anonymous tips.