Having the option of executing actions at any waypoint where the curve size is zero would certainly provide more flexibility. As a software developer, I am guessing that this feature is a difficult one to implement. Currently, setting the “Path Mode” to “Straight Lines” is a global switch for waypoint actions where each line segment may be treated as an individual path with a start and an end. This allows one to execute various options at the start or end of that segment.
However, with curved paths, even if the curve size is set to zero at a waypoint, may not allow the portions between waypoints to be handled as individual segments. If so, to support such a feature would require that as the drone travels along the path, its current position would have to be continuously compared to all waypoints to see if it is close (you wouldn’t be able to test for equality) to a waypoint and then execute any actions defined for that waypoint.
I could be all wrong about this, but this is my best guess as to why actions are supported the way they are.