I think it's important to evaluate WHY your child is doing it and how often they're doing it. I started smoking pot when I was 15, but it was definitely because I was depressed and wanted to feel good. I'd do it maybe 3-4 times a week, and my mother had many talks with me about it how she didn't agree with it, my grades were slipping, etc etc. I quit at the end of my sophomore year because I just wanted my grades to be better so I could get into a good college.
I started up again in college, but only recreationally. Maybe on the weekends every once in a while.
Now, as an adult, I smoke maybe 3 times a week, mostly on the weekends. When I had a job I couldn't smoke during the week or on school nights because it just wasn't something I wanted to do and I felt like it carried over until the next day. I find myself smoking more now that I'm out of a job, and I'm curious if maybe I should just up my anti-depression meds instead. I'm unhappy in general, and can tell I'm self-medicating, but I'm also into more natural methods of medicine nowadays, so I'm trying to figure out what I need to do.
I would say it's worth a talk. Ask them if they're happy in general. If there's some/any hesitation or they say "most of the time," I'd really pause and discuss my concerns for their mental health and happiness. If they really do seem happy and this is a weekend thing, I'd still discuss my concern of it turning into something else and it being illegal, and voice that I'll always be there for them but that it can't be brought into my house.
I think it's possible to be caring and have strict guidelines at the same time. It is a really tough issue, and something I think about a lot when it comes to my future parenting.