Then he goes from fiction to documentary. Why? To tell us that racism still exists?
In a post-Oscar presser he talks about this film being on the right side of history. So I think in some respect he included it for future audiences. So they can see what was happening in the real world at the time this film was made. Because in 30 years people will only vaguely remember how it was... but a clip like this will bring those memories into sharper focus.
He says all this after having spoken about the 30 year legacy of Do The Right Thing and how all of the issues in that film are still issues today. I think he has a lot of admiration for those people who've lead the fight over the years, and he would like to be remembered as one of those people. I think he's thinking of legacy and how he wants to be seen.
At the same time I think he's bumping up against the limitations of film making in terms of how much change he can actually bring about with his work. And things like including this newsreel montage are him trying whatever he can to make the film more powerful. The fact that Do The Right Thing is still so relevant is a testament to the film, but does that not also call into question how meaningful even a good film can be if after 30 years it has made no difference? It probably has made a difference but I suspect Lee must be frustrated, as all of us are, at the pace of change. And that frustration is pushing him to make certain choices in his filmmaking.