You are assuming people will "resee" it. But people need to have seen it for the first time somewhere in order for that to happen. A lot of people don't know this picture, or have idea what impact it has, and if Facebooks censorship remains unchallenged, a lot of important pictures will remain unseen by a growing proportion of people.

This specific people may escape that fate, as it's important enough to be in history books - I believe I first saw it in primary school - but handing Facebook the power to hide important parts of history from a huge proportion of people is dangerous.