I imagine it's because 'processed foods' aren't bad in themselves. That you can add something bad to them doesn't mean they're inherently bad. If someone wants to crusade against bad foods, saying 'processed foods' is the wrong way to go.
It's like protesting against DHMO. Sure, it kills people. But it's not even a good simplification of the whole situation. (Yes, I know that DHMO is H2O, just in case anyone thought I didn't for some reason.)
It's a little disingenuous to suggest that the ingredients of a food are 'something that can be added to it.'
If processed foods tend to be bad (which they do) you can quite correctly crusade against them if you're interested in helping people not to eat bad food.
