On one hand, you expect the result to be an independent resource that can be modified without affecting the source. These expectations may conflict. Unix by default copies the target, not the hard or soft link. Maybe we should take the point of view of the site administrator, whose interest is in keeping down-level clients from messing up the site. COPY may be one of these.
More discussion is needed.
The behavior you would expect is for both the reference and its target to be locked. If only the target is locked, someone could replace the reference with one pointing somewhere else while the target is locked. Geoff points out that even locking both the reference and the target will not guarantee that the same URI will identify the same target resource after the lock is released. So he argues that by default we should not lock the reference at all, but just the target. This is consistent with most other operations on references.