What was Lee's message?
The predominantly black Brooklyn neighborhood is portrayed as a generally peaceable environment with the various races/ethnicities getting along just fine with only minor (and amusing) altercations. Yet it's a tinderbox that doesn't take much to set aflame.
The last act leaves a bad taste. I can't believe Lee had the gonads to be this honest, but he shows why most people don't want to live or do business in black neighborhoods, including many blacks.
While people debate who's right and who's wrong in the story, it's simple: Buggin Out taking offense about something immaterial at Sal's pizzeria is unjustified. If he thinks it's that big of a deal he doesn't have to dine there, plus he can start his own restaurant and decorate it however he wishes. This was Sal's understandable response.
At the same time, it could be argued that Sal should've reacted in a wiser way that turned away Buggin Out's curious anger, rather than augment it. In other words, it didn't matter if Buggin Out's outrage was justified or not, Sal should've assuaged it since peaceable relations with his clientele (and potential clientele) is in his best interests. Meanwhile Radio Raheem made a foolish decision by allowing Buggin Out to negatively influence him.
Why can't they just do the right thing? It's frustrating.
How about doing the right thing by making art that inspires hope, unity and healing for inner city communities? This well-done iconic flick points to the problem, inspires questions & debates, but offers no solutions except... move away from black neighborhoods. That's what Sal & his sons had to do.