I wonder why a person of any nationality can participate in regional championship.

For example, last weekend was european championships 70.3. Rudy von Berg (USA) won. In any other sport (cycling, swimming etc.) only europeans can participate in european championships. The same seems to be true for other regional championships as e.g. ironman texas (for north america).

Does anyone know what the rules for this are? Or should these titles be seen more informal? I don't really mind, but I just find it weird that it is so different from other sports.