Because we see that even though many of the people of Gotham believe that Batman is a murderer (Foley, Daggett) -- there are many (symbolized by Blake, the older cop "you're in for a show...", Gordon, the young orphan) who still believe in the Batman. They believe that he was a force of good.
Kay. That's two people who believe Batman is a murderer and two people who believe he's a force of good. Three, counting Gordon. In a city of 12 million (30 million in TDK) I'm gonna be honest...for me, that hardly represents all of Gotham, even if its supposed to be a presentation of both sides of the issue.
If Bane exposed that the so called, aloof, greedy, apathetic, rich ruling class (Bruce Wayne as the wealthiest in Gotham) and Batman were one and the same -- it would raise serious doubts as to whether the rich were truly evil. Bruce Wayne, the richest and most powerful is Batman? Batman a guy who many still quietly regarded as a hero? This would almost become a rallying point for the people of Gotham to go against Bane...
I think you're missing the point of this part of the movie. Don't listen to Bane's rhetoric...look at what's actually happening.
Bane's whole thing about the wealthy is just a smokescreen. Its just something Bane says to justify topping to existing power structure in Gotham.
I don't think most of the people of Gotham actually think the rich are evil just because Bane wants to punish them.
The criminals Bane breaks out and the existing criminals of Gotham want what the wealthy and priviledged have. The criminals don't care if the wealthy are "evil" or not, they just get broken out of prison and they see their opportunity to have what the wealthy have if they join Bane, and some of them do, and they take what the wealthy had.
Bane sets up the court that murders the wealthy and the elite, and the criminals go along with it because they want the stuff and money Bane promised them.
Its a thin motivation at best, and has nothing to do with the rich actually being evil, or with the general public believing they are evil. Its just an excuse for criminals and bad people to loot Gotham during Bane's martial law.
There is a chance that it could have helped Bane, as those who hate Batman would then hate Wayne more -- but it could have also terribly, terribly backfired and lead to Gothamites seeing that there were rich and powerful people doing good and that there were those who thought that Gotham was worth saving.
Maybe. But this is your conjecture. Nothing in the film shows this, that they would rise up on their own against a man who is evil just because he turns out to be a rich guy who was Batman...when they didn't rise up against that man who was evil to begin with.
It would have been nice to see, actually.
Although, Bane still has a nuclear weapon, though, doesn't he? I hardly think he'd be worried about the people rising up when he could just blow Gotham sky high.
