This is just because Poland as a whole is still completely unfamiliar with the concept of the national ID card being acceptable for EU travel - witness how many of them still take a passport to places like Germany!
Delphiandomine here was not right, as I live in one of the biggest German cities and witnessed many times post clerks unwilling to accept EU id card. One was even complaining that my Polish password has Polish characters which he doesn't know how to write down (ć, ń, ó, ś, ź).
But it's partly understandable, because in Germany they have system of writing German characters with combination of 2 Latin characters (Ä, Ö, Ü, ä, ö, ü can be transcribed as Ae, Oe, Ue, ae, oe, ue).
And variety of EU Id cards is stunning, so it's hard to require from every clerk in EU to know all possible combinations. IMHO it should be unified, for example look at Italian Id:
Finish is more similar to German or Polish, but still layout is quite different: