No professional sniper would ever aim at the head (Oswald notwithstanding). Given the very long range that a sniper would have to cover, and the resultant drop and wind issues to consider, aiming for a small, often rapidly-moving target such as the head would be a mistake.

The chest area overall not only moves more slowly, but also presents the "center shot" target for a sniper that he'd try to hit.

As for the head being left 'exposed' by the glass enclosure, consider that for a sniper to hit a target in the head, he would have to be on higher ground (e.g. a building) and have a clear line of sight. Sniper rifle ranges being what they are (the longest confirmed kill was at 1.42 miles for 35 years until 2002, and is now only about 450 feet more), you can be pretty sure that all relevant buildings within that range have probably been checked. If you consider the shooting angle involved, his chest area is at a much greater risk from somebody standing a reasonable distance away, maybe with slight elevation.