It is in the interest of the photographer presenting a batch of undoctored photographs for viewers to be confident that there are no doctored photographs in the batch.
Photographers who make and label undoctored photographs — and 3p-cp’s who publish such photographs — can keep in mind the “there’s never just one bad peach” principle:
When they find one bad peach in a bag of peaches, shoppers rarely assume that it’s the only bad peach in the bunch.
Similarly, viewers who encounter one photograph that they know (or even suspect) is doctored in a batch of supposedly “undoctored” photographs usually will not assume that that one photo is the only doctored photograph in that batch.
When viewers of photographs discover even one doctored photo in a batch of supposedly undoctored photos, they often will not trust any of the photographs by that photographer. The damage to the photographer’s reputation can last even after the photographer is long gone: see “C” on this page.
The same is true for an information-provider like a newspaper: publishing even one reportage photo that is revealed to have been doctored can cause viewers to mistrust that newspaper permanently (which is why respected newspapers tend to take the “doctoring” issue very seriously).
“Viewer trust” is often all that news organizations have to separate themselves from every other kind of content-provider. Many news providers feel that to abuse that trust would be to squander their most valuable asset.