What you're also forgetting is that by not using their fastpass at the designated time, the line itself goes faster during the designated time. It's not like it can be used twice. I'd be willing to bet that for every person who uses their fastpass several hours after the stated time, there's another person who ends up not even returning to use it. There are also, I imagine, many people who truly believe that you can't use your fastpass if you miss your one-hour window, so they end up throwing the fastpass away or simply not using it.
Even so, I don't think that many people ignore the return time. If anything, most people are like me and they use the fastpass as soon as they can. I think the headache cast members would encounter by having to turn guests away for expired fastpasses would be far greater than the relatively small amount of guests who exploit the fact that the window has no "true" end.
I think they inherently understand that some people are going to return later than the one hour window. But they still print the window on there to encourage people to return on time. I strongly believe that 95% of guests return during that window. The other 5% have an incredibly small impact on wait times. I just can't recall seeing any more of a fastpass "rush" late at night any more than during the day, and I've been to DLR probably around 100 times in the past several years.