My daughter works for the California Department of Forestry and Fire Protection; she's a wildland firefighter. Apparently, from what she has said, they work to put out all fires. Sometimes they need to let them burn because they cannot contain them safely, given wind, air temperature, humidity, kind of material burning, heat, and terrain issues. When structures are involved, like homes, the US Forest Service may not be as ready to protect them as the CDF and may not be as trained in that kind of firefighting, i.e. structure fires. But their first issue is to save the lives of the firefighters.
What is being done in the mountainous area where I live is called "prescribed burns" during the proper weather conditions (sometimes and hopefully) to control the downed wood, undergrowth, etc. -- called "fuel load" here. They burn when the air temperature, humidity, and overall conditions are supposed to prevent a fast burning, out of control, "crown" fire which travels through the tops of the trees.
Apparently the new wildland management technique of prescribed burns really got a boost after the big Yellowstone fire a few years back. It burned uncontrolled and wild because of the built up undergrowth.
That all said, we just had a major fire to the west of us that burned for like 2 months and was huge because they couldn't get equipment and personnel in safely to the rugged mountainous area and the rains finally put it out this fall. The firefighers, both state and US did control the edges where they could.