Hmm, you may be SOL. It's allways up to the owner/manager when it comes to drugs. Ususally a drug free workplace is mentioned several times in documents, so it's no problem canning you for discussing drugs.
Simply talking about it is probably not an issue, but if you were discussing it loud enough for someone to hear, there's gonna be concern on the managers part.
It's BS, but that's life. An unreasonable boss is gonna fire you at some point or another anyway.

On a side note, what do you do (where do you work)? If you were in the service industry, then I can understand what happened. If you work in a rock quarry or something then your boss is a complete douche.

Good luck buddy