I'm not sure how things work in the U.S., but in the overseas airports I've flown through (Munich, Heathrow, Copenhagen) passengers making connections who have arrived from outside the EU must go through security at the connection airport (a major pain in the a$$ at Heathrow). I'm assuming things work the same way in the U.S. as folks arriving at Dulles were told they'd have to pick up their bags before going through security. Actually come to think of it, I did transfer once on an inbound flight from Canada at Newark and did have to go through security at Newark after arriving from Halifax (before going onward to Dulles).
I agree that would normally be the procedure, generally you go through customs at the first U.S. airport at which you land. I was thinking particularly about metal detection for items carried on the person. In this case it's a continuing flight with a change of equipment, so I'm not sure the L.A. passengers would actually go through a security screening in Ohio as opposed to simply walking from gate to gate inside the "secured" area.