Draper is configured to have decorators built by default for every controller. You can change the default configuration with one additional line in your application.rb file...
module MyApp
class Application < Rails::Application
# rest of the config...
config.generators do |g|
g.helper false
g.stylesheets false
g.javascripts false
g.decorator false
end
end
end
Here's the interesting bit from Draper...
https://github.com/drapergem/draper/blob/master/lib/generators/controller_override.rb
Called from the Railtie...
https://github.com/drapergem/draper/blob/master/lib/draper/railtie.rb
Note that you can still generate decorators explicitly...
$ rails generate decorator foo