People are still free to have opinions on the case. The purpose of this legal principle is to put the burden of proof on the prosecution, i.e the government can't simply blame you for a crime and force you to dig up evidence of innocence.

People can disagree even if you're found innocent at trial... just look at OJ's case. The government isn't going to prosecute him again, but "OJ did it" is gonna dog him for the rest of his life.