There are at least four possible reasons:
1) The San Onofre nuclear power plant.
2) March Air Reserve Base (home to aerial refueling tankers).
3) Camp Pendleton Marine Base (borders on San Onofre).
4) Los Angeles and San Diego.
It would probably take a force of several hundred terrorists working in different capacities to work out the details of a plan, and probably only a handful to execute it. One wonders what damage a fully-loaded refueling plane would do to the San Onofre power station and given the prevailing winds, what would become of the Marine Corps base and the cities to the north and south. In one fell swoop
1) San Onofre would be offline (probably for a very long time)
2) Camp Pendleton would be uninhabitable.
3) Large areas of LA and SD would require evacuation.
In my estimation this would result in 100 times the human and financial disruption as that caused by the loss of the World Trade center.
Areas of LA County are so distant from the reactor that there is little that would need to occur in that area.
Although I have not looked for the documents, there should be publicly available PUC documents from So. Calif. Edison which has projections of fallout patterns based on prevailing winds. I think of particular concern for fallout would be Camp Pendleton, but the area is not very populated. Areas of Southern Orange County which are highly populated can be at risk but the likelihood of a major core breech are extremely low.
Edison was researching various fire suppression solutions to protect the reactors. The buildings can repel an assault by a jumbo jet because engineers have already run the analysis. However, the jet fuel fire afterwards is a different matter which they were attempting to resolve.
I doubt that a fully loaded aerial refueling jet could be commandeered. The flight crew would have to commit treason and suicide at the same time. There is no way March AFB has such people.
For those FReepers who are not familiar with the Southern California area known as the South Bay, where Torrance is located, it is a highly target-rich place. In addition to the refineries and LAX, there are a number of companies involved with aerospace work including TRW, Hughes, an Air Force base, Northrup, etc. Just a little south-easterly is the Port of Los Angeles. There is also the huge Del Amo mall complex in Torrance, one border of which is on Prairie just south of the Mobile Refinery. The national HQ's of Toyota, Honda, and Nissan auto manufacturers are also in the immediate vicinity.
If my memory is correct, I believe one or two of Atta's gang lived in this area for awhile before implementing the 9/11 plot. So the danger is very real, even if the national media is not paying attention to anything much beyond the east coast.