Hi Simon:
It's quite reasonable for employers who are hiring candidates for overseas (expatriate) positions to require that they be in good health, and that includes dentition. If an employee becomes ill (for example, gets an infected tooth root, etc.), it can be awfully expensive to have to pull them out, get them looked after, put a replacement person in (or face a downed aircraft, etc.).
I worked for the Red Cross for a number of years in overseas locations - at times for the Red Cross directly, at other times for the aviation contractor that supplied the planes - and I recall that when we did have to medevac out a co-worker, the typical cost of the whole excercise was about $25K, if you include salaries, airfare, medical attention both at the jobsite and once you got them back to 'civilization', etc. Some medevacs cost upwards of $100K just for the plane to get the employee out, although it is unlikely that kind of cost would arise from a dental problem (unless, perhaps, sepsis sets in...)
So, under the conditions stated by the original poster (expatriate work), it's a reasonable requirement.
Michael