The main reason why stock invoiced out to a customer on a Service Order is not automatically retuned into stock when issuing a credit note, is that stock issued to a service order is not always in the same state after invoicing as it was at the time of issuing.

For example, it may have been cut, welded, painted or any other process done to it, which makes it different to the original item code.

For this reason, when doing a credit note on a Service order Invoice, the stock needs to be manually inspected and then manually GRN'd back into stock, and maybe not even be to the original item code.

The option to Return stock issued to a work sheet was done mainly to resolve situations where stock was issued to a work sheet,in error, and had to be revered out and then the correct stock issued.

You would still have to be careful when using this facility, as you cannot Return stock to the original item code if it has been worked on, and the state has changed.