I don't really see how this would make sense as a toggle.
Other things are already toggles. Fleet engagement range? Auto-casts? Those are toggles and they're behaviours. That makes sense. But you click on a ships to attack them, that's an action. This is a modifier to that action. A toggle would what, put you in a mode where every ship click did this by default and now you have to turn it off or use a different hotkey to target a specific ship?Why would you want that? You'd just be making the UI more cumbersome.
Some things are suited to toggles, others are not. It's a good thing to consider as there could very well be cases where they've not made the best choice between what should be behaviour toggle vs a command modifier, but this very much does not seem like an example of that.
Edit: Actually, I'll revise that slightly. More broadly speaking, it's absolutely reasonable to have toggles for fleet firing behaviours, but such a thing would be in addition to a hotkey to do something specific. The same way the fleets jumping as a group vs individually is a behaviour toggle, but you should be able to override it when issuing a move command (does ctril+right-click still do this? Am I remembering that correctly from Sins 1?)