There will always be a bully on the playground. UA is Denver's "bully". I don't mean this is a bad thing. Of course, they're bigger than anyone else, they have more power in the community and they have the most influence on what happens here. Some other airfields,fortunately, have more bullies who clash and bump heads once in a while.
What UA does with the shuttle route to SLC as you brought up, is keep the market competitive. Competition is good for the consumer. Surely, most seats on a particular flight are not going to be at that low fare. I have a strong feeling that F9's overall price range for all seats is going to be lower than UA's.
Another reason people perceive UA as predatory is their frequent service. This is what business travelers want. They want choices and flexibility, not two or three options a day. Don't worry, F9 is doing fine