And some very liberal religious groups decline to say the Pledge for conservative reasons. Many [otherwise decidedly leftist] Quakers do not because traditionally they frown upon pledging allegiance to something that might conflict with their allegiance to God above all others.

I'm not at all religious but I don't have anything against the Ten Commandments themselves, only their form. I would similarly oppose having text from Faith & Practice hung in courtrooms and schools, not because I have anything against it for its own sake, but because not everyone is Quaker and not everyone should be expected to play by Quaker rules.