Typically this kind of ammunition is typically not used for target practice. That cuts a wide swath in the ‘training’ justification. This ammunition is intended to be a very effective and brutal killer.
Although this may be true for the average Joe like you or I, these agencies choose this ammunition for purpose specific applications and they will do it based on expert recommendations, not just off the cuff. A 180gr JHP in .45acp handles much differently than a 230gr round nose of the same caliber. HP ammunition isn't necessarily chosen for it's habit of body-cavity fragmentation, it's more likely chosen because of it's general tendency to fragment on impact and not over-penetrate things like walls, car doors, etc. Not that it's the complete answer to the problem, but in all likelihood, this is a public safety measure against unintended background targets.
If an officer is going to need practice in order to achieve annual qualification, he needs to be practicing with ammunition that behaves the same way every time... this practice eliminates known variables and increases repeatable performance.
I'm not at all a government sympathizer, but most of this adds up to me. I simply don't have enough information to deduce whether the existence or amount of officers is justifiable, but I don't believe they are coming to get me or planning to in the future. We all know a .357sig is no match for what most of us have in our closets at home.