Picard has actual earth produced wine from his family's own ancient vineyard in stock. They usually just don't drink alcohol unless it's a special occasion, but synthol has the same effect, it just wears off faster, so people can drink within 4 hours of a shift.

"there's coffee in that nebula"

They clearly allow it, whether or not most partake is another matter. I'm also not familiar with anything saying they don't allow alcohol, just that synthetic is a widely used alternative