פירוש על עבודה זרה 138:2

If the mishnah had taught only the first case where they were transporting jars of wine, I might have thought that the reason it is permitted there (if it was presumed that the wine was under guard) is that the non-Jew is worried that the Jew will come back and see him drinking the wine. But if the Jew leaves the non-Jew by boat or by wagon, the non-Jew could simply travel a bit away and do what he wants. Therefore, the mishnah had to teach us that even in this case the wine is permitted (as long as the Jew did not tell the non-Jew that he was leaving).