"They drink beer."
Translation:Loro bevono birra.
Imagine the following situation: a man arrives at his friend's house. The host asks the other one to come in, then the other one asks: "What are they drinking"? The host then replies: "They are drinking beer". The sentence in question is exactly like the latter "Loro bevono birra".
I think pulce meant italians would say bevono la birra, not bevono birra.
As far as I remember (from studying Italian 5 years ago) the definite articles (il/la/...etc) are used a lot more often in Italian than in English. They are frequently used when English would leave out the definite article.
There is no question about whether "loro" can be omitted. The subject can always be omitted in Italian, unless it is unclear from both the verb AND the context who you are talking about. (or if you want to emphasise like in your example)