Yes, I feel this is confusing as well. "Beer" can be both a count noun and a non-count noun.
If I'm drinking multiple cans of a single type of beer, the plural is "beer" (non-count noun). If I'm drinking cans of multiple types of beer, the plural "beers" (count noun) would be idiomatic.
I think this sentence is ambiguous, and should allow both "beer" and "beers" to be correct translations of בירות.