It's worth noting as well that the Air Tractors (at least the 502/504 that I'm familiar with), are VERY simple airplanes, with quite basic systems...and this seems to be baked into the design philosophy. Might be as simple as separate starter and igniter switches were seen as the simplest way to implement the start.
The igniter switch is a 3 position switch - start/on - off - on igniter continuous. In start/on, the igniters only come on with starter switch held on. In on igniter continuous, they're on continuous - there is no controller or other logic in the system as in many other airplanes.