Earlier quoted context omitted.
I'd say because without a modicum of knowledge you'd take it for granted. Compare to "images" of dinosaurs where you'll often read "this is what it should look like" vs "we know squat about the color of their skin". And for space pictures, which are 99% touted as "pictures" and not reproductive works, it's even worse. Hell, as I'm writing this I'm tempted to double check what I know about color-correctness of any spa…
Why aren't space photos real? For deep space photos, where we have to wait for individual photons, maybe they're false color, but many photos are taken by putting colored filters in front of the lens and then combining the exposures, so they are actually true color.
I'm not saying they're fake - I am just saying people believe what they see. So a specific or general disclaimer of "the colors might not be accurate" would be cool, if it's the case for a specific photo.