You should see references to the $this->enabled

It may or may not include the code that controlls when it should show in regard to Free Shipping ...

All Shipping Modules should include this ... but not all developers of the modules look ahead to the changes in the core Zen Cart modules to see changes ...

Example:
Free Shipping has automatic adjustment on weight as weight is there or it is not ...

However, those that are based on item or price should us an additional adjustment that allows for Free Shipping for number of items and total price or the shipping is off ...

If you were to add the existing code ... be careful of how it reads, as you want just the IF statement added, not to change the actual line inside the IF ...

Compare flat.php to item.php to usps.php to see the differences ...

I have not looked at the Royal module so I don't have specifics for you ...