I think you all are missing the point. It's not the drinking or not drinking, it's not about whether or not other slaves, other Dominants, his other slaves, etc. are present. It's about this piece of the question:

Before he can even ask,...

She presumed. She made a blanket announcement based upon an assumption (that he would have asked/expected her to), she made a big deal about it in front of others, and she embarrassed him and made herself look like a 3-year-old who doesn't know how to act.

What was she trying to prove?