I believe they have a high standard: you can doctor a picture, as long as nobody can tell it. An amateurish job like this, which make people realized that it was doctored, failed their standard, so they let him go.
You've nailed it; they didn't can him for doctoring an image, the canned him for being lousy at it.
Lots of examples here:
http://www.cs.dartmouth.edu/farid/research/digitaltampering/