Gentile beer is forbidden, and the rabbis say that his is because Jews and Gentile might intermarry after meeting over beer. The rabbis also mention guards and their usefulness in keeping items kosher. Beer in the end is deemed kosher for Jews to drink, even when taken from Gentiles.
At the end of our daf, the rabbis note that people who ate forbidden foods might also be hurt by the venom left in those poorly supervised foods. Particularly susceptible would be those who are ill.