פירוש על עבודה זרה 62:4

According to one opinion, drinking beer with non-Jews encourages fraternization with them, and that is why it is prohibited. In other words, not allowing people to drink together is a good way to prevent them from marrying each other. The other opinion holds that the problem is that non-Jews leave their beer uncovered, and it might be susceptible to snake poison. The problem with that opinion is that Jews also let their beer remain uncovered. The answer is that if the beer is left so that all the sediment falls to the bottom, then the water on top would be subject to the rules of uncovered water.