I think that is a bad decision. It is NEVER acceptable to say "the police is" in English. Duo should not be accepting incorrect English translations, even if there are a lot of non native speakers that will be confused by this. There are many things in Polish that confuse me, including some issues with collective nouns that aren't collective in English and vice versa. That doesn't mean my ignorance of the language should be accepted by Duo. Would you accept bad Polish from me just because the correct translation confuses me?
because we teach Polish here
Yet this course requires using English all the time and then some. Having finished the tree I get laughably small amounts of EN-PL translation tasks during the practice sessions, especially compared to the Polish to English course where I get to type my answers in Polish most of the time and actually practice the language more than I'm allowed in here.
Witowt, The "audience" of the Polish to English course is "mature" and so the questions are deeper but they cannot be in Polish, so they end up as "EN grammar nitpicking" as you say. In the other forums, unfortunately, there is a lot of very simple (trying to be nice here) questions and comments, so once you filter those out you can learn something. I found it very entertaining to do a reverse tree, especially reading the comments when I finished my Spanish (for EN) tree and moved onto Spanish for French speakers and it helps me a lot to recover/remember my French.
OK, all these comments and the Internet forums have persuaded us that "are" is the only version acceptable in proper English. Therefore none other version will be now considered "the best option" and suggested to the learners.
However, we think that the number of non-English-natives that would start reporting versions that treat police as singular could be so high, that we're gonna let it slide and still accept them even though that's not good English. But again, the programme won't suggest it anymore, only accept.