1- They have cited some hadiths, but these hadiths don’t say that it was done in public. They just assume it was in public because the hadith would say something like “the Imam cursed in his Qunut in the masjid.” The hadith doesn’t say the Imam did so loudly or in Jama’a. It just says he would curse them in his Qunut. So it’s not evidence it was publicly heard by everyone.
2- They say we must always say the truth regardless, and since they are enemies of God we must curse them publicly—even if we don’t have evidence the Imams did it, since the Qur’an tells us to curse the oppressors