I think you will have to decide how you feel about it and then communicate that to her.
If you are against pot and don't approve of her using it, then I think you will have to say so. If you want to take it from her but are choosing not to because you are hoping she will reach a decision on her own, then I think you will have to say that, as well.
My best suggestion is to talk openly and honestly with her.
You might tell her, "I don't approve of pot and I don't want you to use drugs. I won't take it from you, but I am hoping you will think about it and realize it is something you can choose not to do."
Then ask her how she feels about her loss of privileges. Is she willing to give up the pot in order to regain any of her privileges?
You and your daughter both need to understand that trust is the most important thing in your relationship. Thank her for being honest about the pot. Let her know that you cannot be with her every waking moment, and that you realize she can tell you one thing but do another - especially when she is away from home.
Let her know that you love her, even when she makes mistakes. Tell her that your job is not to make decisions for her, but to teach her how to think for herself and how to make her own decisions - decisions that are right for her, not for her friends (or for you).
Hopefully, she'll find her way back to your values pretty shortly.
Good luck to the both of you.