Hi Santoo
Scrap is generating during production process so when you confirming activity of your output qty, the activity hours/unit is confirmed for whole process time/duration..so my question is why do you want to charge the cost of activity only on good products? Logically it should be for both because you are getting good and defective/scrap both in the same process..so I don't agree that activity cost should not be charged to scrap if salable or used in recycle.
Now, if you want that scrap will not be charged any cost then define it as a arising material in BOM and make goods received through mov type 531 and book your confirmation on good products only.
Kamal.