No, allowing ADRs shortly before closing has nothing to do with the party. All the parks have restaurants that do this, every day of the year. We often make reservations that keep us in parks well after closing. It's one way we maximize park time - park closes at 9:00, make an 8:45 ADR.
Two very different things are discussed in this thread - people who are trying to "steal party atmosphere" by sneaking around with no wristband, and people who just want to use every allowed minute of a shorter-than-average park day. Conflating the two of them and acting like they are the same thing is just dumb.
I wouldn't try to steal any party experiences I didn't pay for. But I have been there on party days. And if I am on Main Street at 6:45 and I decide I want to ride the Mine Train one last time before heading out, I am going to do so. And that isn't breaking any rules or making a questionable moral choice. I love that Disney has the "if you are in line" rule. It's one of the cool things about going there, that victorious feeling when you make it back into the queue in the nick of time for that one last ride.
I once got in a two-and-a-half-hour line at
Disneyland at 11:59pm when the park closed at midnight. We got on the ride.