It may be just as easy to alter digital photos as it is to alter traditional photos, using a scanner, but the difference is that a lot of people work directly on their digital pictures, and don't save the originals. With film, when you digitally alter the photo, you aren't actually changing what's on the negative, you are altering a digital copy of the original (or a copy of a copy of the original if you scan in a print rather than a negative.)
Slashdot Mirror
It may be just as easy to alter digital photos as it is to alter traditional photos, using a scanner, but the difference is that a lot of people work directly on their digital pictures, and don't save the originals. With film, when you digitally alter the photo, you aren't actually changing what's on the negative, you are altering a digital copy of the original (or a copy of a copy of the original if you scan in a print rather than a negative.)