I get the impression from what I have read so far about digital photography that all pictures have been somewhat modified in a photo editing program before being displayed in magazines. I would have thought that a picture taken by professional photographers would not need photo editing.
It depends upon how we define “modified”. Technically, you are quite correct. At minimum, basic adjustments such as white and black levels, contrast, saturation, etc. are usually required to produce a good magazine print. But that’s not really any different than what we had to do with film. it is also normal to crop images, especially if the camera aspect ratio is different than that magazine page or other space in which the print is required.
The extent to which other editing is done dpends upon the image and the nature of the publication. For example, photojournalists are generally ethically prohibited from retouching photos, while glamour images may be extensively retouched.