Rails - Feb 2014 - Why does calling .delete_all on has_many relationship nullifies foreign keys.

I have several years of Rails expirience and this little bit ALWAYS gets me:

class Foo < ActiveRecord::Base
  has_many :bars
end

class Bar  < ActiveRecord::Base
  belongs_to :foo
end

# after creating some foos and bars

foo.bars.delete_all #=>
UPDATE bars SET foo_id = NULL WHERE foo_id = $1

Yes I know I can set :dependant => :delete_all

but what's resoning behind this default behaviour? Is it just a legacy 
stuff?

I'd expect foo.bars.delete_all to act the same as Bar.delete_all or 
Bar.where(foo_id: foo.id).delete_all regardles of the :dependant option.

anyone knows why the default behaviour here is to nullify keys?

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