I would believe that generally, the point that this is acceptable for a mod is the same reason they are entrusted with the powers to begin with - As a mod, you are expected to be interested in a peaceful forum more than the average user. The classic reason for locking a topic is that the discussion in there has decayed into an outright verbal battle and a civilized discussion can no longer take place. However, among the limited group of mod-privileged people, this need not be so. If a mod states his opinion in a civilized manner in a locked thread, the originally warring factions still cannot pick this up to fight about it. Quite similar to a Wiki really, where edit wars often result in a simple protection of the page. Sysops are not prevented from adding changes, but the fighting is quenched.
Plus, the mods and admins have bigger stars than you.