Given that religion is based largely on faith, the belief in unproveable tenets, destructive criticism is pointless; it often devolves into a meaningless argument based on "I'm right because my religion says..." sent back and forth between participants in an increasingly heated fashion. In addition, many people base a great deal of their self-concept on their beliefs, and deriding those beliefs derides the person as well.
And is this is the biggest reason why criticism of religions is so taboo. Partly because of how it is being given and partly because of how it is being received...and in either case, it is usually because the people are too emotionally involved to have an open mind.