Is it true that Shia Muslims consider non-Muslims as impure? If that is the case, what about those non-Muslims such as Hindus and Sikhs who participate in Muharram congregations and give food to participants?
Allah, The Glorious says in Quran (9:28) : إنّما المشركون نَجَس INNAMA ALMUSHRIKOON NAJAS. Verily the polytheists are impure. This a Quranic fact which all Muslims should follow.
Those who participate in Muharram believe and love Imam Husain (AS) being a divine leader of Truth, which means that they are not really polytheists even if their families are been classified as non Muslims. Their noble act in commemorating Imam Husain (AS)'s martyrdom reflects their undeclared faith in his values.
Allah Knows what is in the hearts and the acts usually reflect what is in the heart. That is why we take the acts who love Imam Hussain (AS) as an indication of their faith in him and his faith which is Tawheed.
Wassalam.