Indeed, voters have every right to criticize what a candidate does. However, voters also have a right to choose their candidate(s) based on their religious beliefs, especially if that is what matters the most to them. Just as you would not look at a candidate's religion, others have chosen to consider it otherwise.
Why should they go by your standard and exclude the religious beliefs of the candidates, anyway? You have to realize that even the notion of excluding the religious belief of a candidate is itself based on a religious belief, highlighting a secular humanistic outlook on how to vote (the belief that religious beliefs aren't important to a candidate). Yet, you would enforce that religious criterion upon Christian voters, and that's just not right, if we go by your standard.