I believe 'kan' is used to tell you it is against some kind of social norm, rather than a legal or physical law, so the whole sentence is kinda subjective. There is nothing stopping you from mixing wine and beer (as opposed to mixing water and oil) and you have full permission from anyone if that's your thing, but if you were about to do it in a group of friends or something, I think they would probably react like this. Just a theory though, not a native speaker.
I'm not a native speaker, neither of swedish nor of english. So I am surprised to see that 'kan' is used in swedish and that 'can' is used in english. The speaker does indeed not try explain a law or a possibility, he wants to warn against mixing beer and wine. In english I 'd use 'should' in this context. In swedish 'skulle'?