Or because as long as people are OK with that bit of intrusiveness every time they travel, they'll be more accepting of other restrictions on their freedom as well.
I think the other theory is more plausible. I don't think there is some massive conspiracy to increase government intrusion for its own sake, such that there are deliberate attempts to desensitize people to it. Don't get me wrong, there is plenty of intrusion all the same, but the motive tends to be more directly tied to somebody with a stake. They don't want to monitor what you download just to do it - they want to monitor it because they get bribes from Hollywood and Hollywood wants to stop movie downloading, etc.
Whenever something bad happens there is ALWAYS a blame game. Actions that were perfectly reasonable get questioned if they were somehow tied to the chain of events. If somebody blows up a bus full of kids while it is stopped at a traffic light, some idiot is going to propose that school buses should have flashing lights and be able to drive through red lights to reduce the opportunity to attack them with RPGs.
It seems like a 100% certainty that at some point in time terrorists will attack another US airliner. Security can make that very difficult, and maybe it will happen in 10 years, or maybe it will happen in 50 years, but sooner or later somebody will figure out a way to do it. At that point, everybody is going to be pointing fingers at anybody who voted no when the bill came up that would have instituted some control that would have prevented that particular incident. Never mind that there are countless areas where you could tighten security and if you tightened them all we'd be flying naked on planes with our baggage in separate planes after having all gone through full body cavity searches.