I don't think you can blame the particular agent; there is no reason that the agent would not want to give you the info, unless they've been instructed not to. (I agree that the "security" excuse is BS, but I don't think it's right to blame the agent who's just doing his/her job.)

I have noticed that all sorts of information that routinely used to be given out now is withheld for "security" reasons. Last month, while driving to BOS to pick up my parents, I heard on the radio that an AA flight on its way to BOS had been diverted to RDU because of a bomb threat. Although they gave enough details on the radio for me to determine that it wasn't my parents' flight, they did not indicate where the flight originated from. Just out of curiosity I stopped by the ticket counter while I was waiting for my parents' flight and asked. The agent very politely indicated that they could not give out that info. Which, as the OP pointed out is stupid, because it's easy to figure out. I went to the arrivals board and saw that one flight was four hours late inbound from DFW. Also, there were reporters all over the place asking people if they were waiting for passengers from flight XXX (which was the DFW flight that was four hours late).