It is not surpirisng people think it is religious and in some ways it is certainly dis-ingenuous....the BB talks of a Higer Power, sounds pretty open doesn't it? As you read further it says something like "the Higher Power which is God", so suddenly the open concept is not so open. Then it becomes clear by use of language that they mean a Christian God, "Father of Light", "Creator" etc.
In the rooms of AA people talk of using trees or a doorknob as a HP but the BB clearly intends those things to be a starting point that will lead you to God eventually.
Some people try to excuse this narrow approach to spirituality by saying that the authors came from a culture where the Christian God was the only real spiritual concept around and that might be true but the fact is that if you want to "do it by the book" as many AA people pride themselves on doing then you HAVE to believe in a God that is anthropomorphic and is clearly Christian based.
It is no surprise to me that AA does antagonise people and people are put off by the God word..... I say this as someone in AA, who likes AA and works the Program without an anthropomorphic God.
You can't have a book that mentions God and it's synonyms hundreds of times and be surprised that people think it is religious.