Why can you not have a foreign key in a polymorphic association?

Why can you not have a foreign key in a polymorphic association, such as the one represented below as a Rails model?

class Comment < ActiveRecord::Base
  belongs_to :commentable, :polymorphic => true
end

class Article < ActiveRecord::Base
  has_many :comments, :as => :commentable
end

class Photo < ActiveRecord::Base
  has_many :comments, :as => :commentable
  #...
end

class Event < ActiveRecord::Base
  has_many :comments, :as => :commentable
end