This is the first I am hearing of this. This particular example is clearly "photoshopped." You can see the aspect is off, meaning the lettering looks centered, but the headstone has a slight angle. I have seen instances where later ancestors provided or replaced a stone, but there will still be something of nature or weathering to the stone. Even a bird dropping.
So, are you saying that people are making up fake headstones and posting them at Find A Grave? I wouldn't care if it were marked as an artist's rendition or an obvious hand-drawing. Better to put a photo of the person than something like this that will mislead some into thinking this headstone depiction could be a source, however flimsy.