The scenario that
@DiannaVM describes above is one possible reason why: if your FP window hasn't opened yet, you don't scan through and the FP isn't "used" for your early walk-on, you could get off the ride and then jump right back in the FP+ line for a second go using that same FP. Since the stand-by line for FoP at 9 am is routinely shooting up to a 90 to 120 minute wait at opening, I can see why this ride might have a stricter policy on early arrivals.