Rails core - Mar 2014 - boolean validation

I'm relatively new to Rails and was wondering what the reasoning behind the 
following is:

I have a boolean field processed that I want to set to false by default:

  before_validation(on: :create) do
    self.processed ||= false
  end

  validates :processed, presence: true

This only works if I modify it to the following, which with my knowledge 
don't find intuitive:

  before_validation(on: :create) do
    self.processed ||= false
    true
  end

  validates :processed, inclusion: { in: [true, false] }

Is this a design choice or is it simply too hard too implement 
(Object#blank etc)?

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