More often than not, they are wanting them back or ask that they be destroyed.
Why? because folks who would get the "If they were so bad to need replacing anyway..what good are they" parts back often turn around and sell them
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So DC may get a black eye a second time.
Initially DC may want parts back to determine failure modes and to make product/process improvements. Later, after they have what they need, they may not want the parts back, but request them to be detroyed so they do not go the "sale" route. If you were them, would you do any different?