.js files fallback on .html if view is missing

Notice that in the below given case I do NOT have index.js.erb.

  def index
    @users = User.all
    respond_to do |format|
      format.js  { render }
    end
  end

  #views
  index.html.erb (present)
  index.js.erb (not present)

Now if I request http://localhost:3000/users.js then I do not get template missing error. Rather I get index.html.erb file. And that is because of this code https://github.com/rails/rails/blob/master/actionpack/lib/action_vi...

Any particular reason why the code is the way it is. There is a corresponding test also for that line of code.