I have realized that all Batman movies have a weak 3rd act.
BB - Microwave emitter + fear inducing weaponized hallucinogen deployed in the Narrows leading to some poorly shot action scenes and general pacing
TDK - Boat scene with shitty extras acting and the fight in the building plagued with typical Nolan action sequences where you can't see shit. Plus that whole act along with Harvey's descent into Two Face was not paced well.
TDKR - The whole action climax was not nearly as epic or as big of a pay off as the movie had been building it up to be. The whole bomb sequence was probably the worst part of the whole movie. Pacing and editing was really shoddy here compared to the rest of the movie.
Overall BB has the least worst 3rd act because really the only thing it is guilty of is having a different tone from the godly 1st act of the movie. It succumbed to many comic book tropes but it was decently enjoyable as were the TDK and TDKR 3rd acts... you just had to shut your brain off at those segments.
All 3 movies had a GODLY last 5 minutes though... DAT Joker turn + "And you will never have to", Batman riding off into the night with the Gordon monologue and of course the TDKR ending with Bruce finally not needing Batman for himself. Amazing stuff.
Serious question: Why wouldn't they? It's not a throwaway scene either. It is the first interaction Gordon had with Bruce Wayne. Even if they don't remember the actual scene in question, when Batman says a hero can also be "a man who puts a jacket on a young boy to comfort him" I don't think anyone who is of at least average intellect and who is actually paying attention to the film and characters will not understand what he is saying.
A lot of people haven't actually seen BB as compared to TDK and most likely TDKR. It is no surprise that the bulk of flashbacks came from BB instead of TDK. When I first saw the movie I could barely even make out what Batman was saying there (I heard hero, coat and boy) so the flashback was helpful to me even.
It is definitely fine to put that flash back in fact Nolan is known for putting in flashbacks from the SAME movie in those types of moments. That's just his way of things.
Plus people here overestimate the general audience's capacity to figure out stuff even as simple as this. You have no idea how stupid some of the questions were when people came out of the movie... some people actually thought Ra's was alive if it weren't for that cheesy fade out sequence.