Found this on another sites forum.
By using your FP beyond the window, you are not "getting away" with something, you are not "breaking rules", you are simply using the FP as allowed. When I use my FP does not affect crowd control at WDW or the attraction. My use of the FP does not have any impact on 1)the number of people in the park, or 2)the number of people who can ride an attraction in a day. FP's are not intended to move you to the front of the line, they are to allow you to spend your waiting time somewhere else (like buying food or merchandise). Disney is a business and they understand that the only time you cannot spend money is when you are riding and when you are in line. FP is a win-win idea that reduces your standing in line time. If I use my FP during the time window, someone waiting in the standby line will just have to wait a little longer. If I use it later that day, that same person (and many others) will simply get to ride a little sooner. Either way, the same number of people still ride. Complicated? NO - just simple logistics and ride management.
I have do have firsthand information concerning "late" FP use. My cousin, Scott Bowden, works in Anaheim as a Senior Vice President in the Department of Planning, Revenue Management and Strategic Pricing - he and I have had a few conversations about this issue. It has now become a joke because whenever I call him, the first thing he says is "Fastpasses do not expire, what's up?"
Additionally, a father in our first China adoption travel group (Mike Lewinski) works for the outside firm in Buffalo that actually helped Disney design and implement the FP (they make some of the components in the machines and in the software). He is very familiar with the logistics of the FP and how they work to provide better guest movement in the parks.
To answer the question about why they print an end time and not simply the earliest time you may enter the FP line:
Because someone realized that people need guidelines. Seriously, it is because having a start and finish gives an appearance of a "reservation" instead of just giving you a time to return (which just seems like you're still waiting, just not in the line). People like an appointment. I am not kidding – it is no more scientific than that.
I do hear the argument that if everyone holds their FP until later it will clog the system. Consider the following extreme example:
5000 people collect FP's for Space Mtn in the morning and ALL of them choose to return after their window. The people in the standby line just have to wait. Still, the same number of people ride the ride and it never stops - it runs at capacity. BUT - for every one of those people who hold their FP until later, 1 person gets to ride earlier in the day.
Simplest example: you and I are the only people in the park. I use my FP at 10am(earliest return time printed on the FP), you (standby rider) have to wait. OR, I use my FP at 5pm(well after the end of the stated window) and you were able to ride anytime before that - with me not getting in your way.
If you don't believe you should use the FP's past the time, then don't. The fact is this: If I choose to not ride during the 1 hour window, it only means that someone else will fill my magical spot on the ride (someone from the standby line). If I did enter during the 1 hr time, that same person (theoretically) will ride AFTER me. None of this is intended as a rationalization, it is just basic theme park logistics. FP's do not expire (on that day, of course) and I know that for a fact.
