"The policemen are men like us."
Translation:I poliziotti sono uomini come noi.
When you use it and when not - the rules are more or less like in English. But, here you cannot use "GLI poliziotti", but "I poliziotti" because the word poliziotti begins with a normal consonant. When the word starts with any vowel or with some or group of letters as S+consonant you use GLI. Examples: Gli Uomini (u - vowel) Gli STudenti...
In short, when you use L' or LO in singular case, you use GLI in plural.
In this case, it's quite like in English... you don't say they are the men, you are defining policemen as belonging to the general group of 'men', if this makes sense. Look at examples like 'they are businessmen', 'you are a lawyer' and so on... We define 'they' or 'you' as something, not the other way around.
I hope it helps
I saw many examples in the course where the suggested translation is indeed "police officers", but "policemen" or "policewomen" is also accepted. So Duolingo already does what you suggest.
However, in a highly gendered language like Italian it is not possible, and I suppose the authors also want us to learn the usage of "il poliziotto"/"i poliziotti" and "la poliziotta"/"le poliziotte" as well.
Sometimes you need to know the terms for only male police officers and only female police officers, e.g. when talking about statistics saying that policemen are more violent than policewomen. But when talking about police officers in general, I fully agree with you.