is there any specific reason why there is no strategy specified for on_delete on resource owner foreign keys for oauth_access_tokens and oauth_access_grants tables?

I see that the example in documentation recommends to specify :delete_all for dependant option:
https://github.com/doorkeeper-gem/doorkeeper-provider-app/blob/master/app/models/user.rb#L16

but, the database migration, does not specify any at database level:
https://github.com/doorkeeper-gem/doorkeeper/blob/1cd750b2bfc68b641c11a8ec0297feb3825eb97b/spec/dummy/db/migrate/20151223192035_create_doorkeeper_tables.rb#L66

Below is an example of code showing the issue:

class User < ActiveRecord::Base
  #...
  has_many :access_grants,
           class_name: 'Doorkeeper::AccessGrant',
           foreign_key: :resource_owner_id,
           dependent: :delete_all # or :destroy if you need callbacks

  has_many :access_tokens,
           class_name: 'Doorkeeper::AccessToken',
           foreign_key: :resource_owner_id,
           dependent: :delete_all # or :destroy if you need callbacks
  #...
end

user = User.create
application = Doorkeeper::Application.create(name: 'test')
Doorkeeper::AccessToken.create_for(application: application, resource_owner: user, scopes: '')

user.delete # => ActiveRecord::InvalidForeignKey 
user.destroy # => true

I would expect destroy and delete both work.

Thank you!