פירוש על עבודה זרה 62:8

According to Rashi, Marguan is a city in which non-observant Jews reside. R. Shmuel b. Bisna would neither drink theie wine there nor their beer. Theoretically, there is no reason for him to have been so strict with the beer. But the fear seems to have been that due to their lack of observance, he should act even more strictly than necessary and not even drink the beer.