Adding the ability to schedule additional FP's to an already existing app would substantially reduce the need and the labor costs of having multiple CM's with iPads at kiosk locations to process overflow; it could also reduce the potentially negative visual impression and guest frustration of lines to avoid lines. Yet the decision so far has been to not add this self-provisioning feature.
Disney knows that they are doing.
This is something I posted about when we returned in May.
Indeed I am quite sure Disney knows exactly what they are doing. Its really very simply. By forcing people to move to, wait for FPs, they are, essentially decreasing the rider demand (and dissatisfaction if forced to wait) by taking people out of the queue even for those few minutes.
This allows FP+ to achieve 2 goals simultaneously 1 - attraction efficiency maximization - steering crowds toward the under utilized attractions (the ones with FPs available) and 2 - decreasing the overall rider demand - by putting in the little time sink of having to walk to and wait for FPs.
Smart really, though not exactly geared to helping out the guest necessarily (though you could spin it that way).
But I am pretty sure this is why Disney has not made it available on the App.
Doing so would require very little code. As was explained to me the Kiosks work on a parallel but very similar system, if they could do it, so could your app, and actually the Guest Relations CMs (the ones in the Plaid vests/shirts) work on another, similar but different, parallel system. All three accessing the same main system of FP management for availability, all with different access rights and FP distribution software.