The "locking" of the time zone is done to preserve the integrity of the timestamp itself. If the timestamp setting could be varied, it would allow third parties to claim that you had altered an entry (i.e "re-writing history" by setting the timezone ahead, making an entry, and then later setting the timezone back.

In short, the locking of the timezone is for your protection. It prevents anyone from claiming that the settings were manipulated so as to allow you to falsify an entry. We feel that absolute credibility in court outweighs the convenience factor of being able to set the timezone to your locale.