I think there's just too much hand-wringing over the whole thing. IF I want another drink, I will accept a drink from someone, but I don't believe that confers any obligation. If I don't want another drink, then a smile and "no thank you." are sufficient.

On the other side, I don't recall ever having bought a drink for a stranger, but if I did so, it would be because I liked his/her face, and I would not expect my doing so to imply ANYTHING.

Short version: whether giver or receiver, be gracious.