There's a way to game that system.
Book your first FP for nine days *before* your arrival date. If the system were to allow ten days of booking, you now have opened up the window for your actual arrival date 39 days in advance. For a guest not staying on site, the system doesn't know that they're not actually arriving on that first date.
Intentional or not, this seems to be a rare instance of Disney actually preventing a loophole rather than discovering and closing it later.
This is already happening in several ways APs, Onsite stays, etc, Its called "walking" a reservation
For instance if you are planning on going from May 10 - 16, pretty easy to book a reservation from May 6 - 16. or May 1 - 10, then book your FPs for the actual dates you want, and call in and modify your reservation stay dates as you like. Potentially this might mean calling in each day and walking your reservation day by day so you can book your May 10th FPs on May 1 ... May 2 you call in alter you dates by a day, book your May 11 FPs, so on and so forth. This has and is being done.
This is NOT why the system is set up how it is.
If it were, there is a SIMPLE solution, you make the Ticket "Active" as soon as you book an FP. Meaning whether you show up or not, you burned a day of tickets. Now, I can see that causing issues as well. Imagine you are planning on flying in and using a ticket on arrival day, that night ride some mountains and catch wishes ... but your flight is delayed, should that cost you a days ticket, no. But you could say, limit your FP window, so if you start and FP window on May 1, to really book until May 10, as an offsite guest your FP window wont advance again until 30 days before may 10. So on April 1 you can book May 1 - 10, but if you want to "walk" your reservations, after you initially book and open your FP bookings for May 1 - 10, you need to wait until April 10 to book May 10 - 20. REALLY simple server side solution, wouldn't negatively impact 99.9% of offsite guests, would positively impact 99.9% of offsite guests booking FPs.
And seriously, if I can think of this in 30 seconds sitting here, I am really hopeful that a multi-billion dollar company could figure this out with a little effort.