I think that folks are just saying that since there is a return time listed the FP is valid those times, use outside of those times is therefor "breaking" the rules....
This is consistent with many factors in everyday life. Coupons at the grocery store are valid until the date listed, sales at the mall are good until the times listed, your movie tickets are good for the time show on them and not for a movie that starts 4 hours latter at the theater, your airline tickets are good for the flight that takes off at the times listed and not hours latter, you football ticket is good for the game listed and not next weeks game etc....
It is a mindset that many have and it has it's validation in everyday life IMO. I think even Disney at some point believed in it or they would not have printed the return time on the FP at all.
I have NSSHP tickets for the 13th of September, I am not going to skip it and try to use them on the 16th of September as I know they will not work. I am not going to show up 1 hour late for my ADR and expect them to be honored. I think FP fall into this same category in many peoples minds.
Heck just because the police do not pull you over for driving 50mph in a 45mph zone does not mean you are not speeding, so just because you can use your FP late does not mean in itself mean you are not breaking the "rules"
As I said I think they should be enforced but am not militant about it, I do think the folks saying it is rule breaking have a valid case to argue...