Often they are motivated by fear of confrontation. Sometimes they immaturely assume that confrontation is unloving. Both responses are wrong. They are also fruitless. God requires that we warn our neighbors, but that we do so in love and with truth.
Christians are more likely to err on the side of being too soft than too hard.
It is easy to cover our fear of confrontation by telling ourselves that in doing nothing we are loving. Love must be tough. The gospel, by definition, is a matter of telling. It is communicated by word. Yes, our actions do sometimes speak louder than words.