Many of the famous vineyards are open to visitors, but tasting is strictly for serious customers who show a clear intention to buy.
All visitors to vineyards can go to tastings, even if they have no plans to make a purchase. 
mnli hypothesis: All visitors to vineyards can go to tastings, even if they have no plans to make a purchase.  premise: Many of the famous vineyards are open to visitors, but tasting is strictly for serious customers who show a clear intention to buy.