Presuming innocence doesn't make him actually, literally innocent. The reality of what happened is what it is, regardless of whether anybody can prove it to a legal standard.
Also, when you say "We tend to presume innocence", that's not really true. It's true (or is supposed to be true) for the government and people who want to be civically responsible, but a whole lot of the general population does not actually think this way. People think OJ did it, have various theories about who killed JFK, etc. People read the news about somebody accused of murder and think "yeah, that guy probably did it."
Also, when you say "We tend to presume innocence", that's not really true. It's true (or is supposed to be true) for the government and people who want to be civically responsible, but a whole lot of the general population does not actually think this way. People think OJ did it, have various theories about who killed JFK, etc. People read the news about somebody accused of murder and think "yeah, that guy probably did it."