There is an aspect of truth in what you reference, but in my opinion the spin in the major network's news delivery is almost exclusively anti-American. Fox may report the same negative stories, but the delivery from them is matter of fact, perhaps even expressing lamentations if those facts ran against conservatism. Rather's delivery for instance, would deliver that fact, but would also attach a negative connotation regarding U.S. policy, almost exclusively anti-conservative in it's scope.
The negative story does attract more attention. I do believe that FoxNews reports too little good news, but I do not see them in the same light at all.
When it comes to the evening network news, these men do not share our values. It's as if they were working for the old Pravda. I used to listen to English broadcast versions of Pravda. I used to listen to English versions of Cuban and other anti-Ameircan stations over shortwave radio. Our network evening news is an exact reproduction of what they used to do. I'll bet Radio Cuba and the network news today could be written by the same dopes.
I also suggest why are the commentator shows mostly about grinding the axe about the lack of positive stories rather than presenting positive stories. My guess is there's more audience reaction to grinding the axe than there's desire for positive stories, even among the faithful. Even Rush, Hannity, Medved, Michael Reagan, Hugh Hewitt, and on and on feature little if anything that's positive. It's most likely complaining about the lack of positive news. But then again, they present little if any themselves.