פירוש על עבודה זרה 144:7

The Talmud here refines the prohibition of wine poured by non-Jews, which was created above by R. Sheshet. This is considered to be a rabbinic decree, an add-on if you will to the main prohibition of yayin nesekh. Since it is an add-on, the rabbis can shape it the way they want. So the rule is that if the gentile pours the wine, what comes out is prohibited but what is inside is permitted. This, by the way, is why in kosher restaurants non-Jews do not pour wine for the customers. What comes out of the bottle is prohibited, even though the non-Jew never touched it.