I'm with you. WHY shouldn't an AP holder who also books an onsite stay get both? She's not asking to get them at the same time (double FP+s), she's just asking to get the AP benefit AND the onsite benefit. I bet if you put it to the Person In Charge Of This they would say "hmm, of course they should get both, don't they already?" and then they'd go talk to the 19 year intern in Seattle who programmed the FP+ stuff and figure out how to make it right. The issue ...as always...getting to the Person In Charge Of This.
I'm also with you. Because that's how we are when we buy APs. Which happened 3 years at
Disneyland and 2 years at WDW, and now we keep APs for Universal because there's still serious value in that.
If I had an AP for WDW, though, I would want my benefits for our onsite stay (no doubt happening because I booked yet another rundisney event, which I have yet to enjoy in ANY way, but I keep booking them hoping to have the same good feelings everyone else does!) AND after the WDW stay I would want to be able to book a couple FP+s for the evening at WDW, after finishing out our day at Universal (while staying up there. They would probably get dinner-money from them, some walking-around time, a few rides, and then we would leave. I don't see the downside for them for allowing this.
WDW doesn't recognize "split stays", they see it as two separate events. They shouldn't see "what happens before you checkin or after you checkout" as something to interfere with "what happens during your onsite stay".