I had a similar experience in a food court in Bangkok. My wife and I bought our food and sat down at a table that had some bagged food on it (since there weren't any clean tables available). Assuming it was someone's leftovers that they neglected to trash, we just moved the food to the side since there wasn't a rubbish bin nearby.
A few minutes later someone came up and angrily told us that was his table and it was his food, and he demanded we find another table.
Really confused but not yet learning our lesson we moved to another table and moved some more food aside. Another man yelled at us, this time even angrier.
Seems it's pretty common for people to buy one thing, take it to the nearest empty table to 'reserve' it, and go elsewhere and get more food.
I guess when you're in someone else's country you just have to realise that you probably are the asshole and apologise, even though it doesn't make much sense :S
Regarding the original question, the couple that arrived early should have "claimed" the two empty chairs by placing jackets or some other items on them. Failing that, they should have been alert to the "older" man who put his jacket down on it and immediately stated that the seat was already taken.
P.S. I specified older generation because the parent specified "older man".
Whenever I'm out somewhere with someone else and they set anything down on a table (jacket, hat, notes, wallet, anything) and get up to use the restroom, they say, "Make sure no one takes anything." I say the same.
I'm not sure if this is just a generational thing that follows from "stranger danger" or what.
I was in the US (NYC) last year and went to a music event at a bar. My girlfriend and I were there an hour early, so we grabbed a table of four near the front (we were meeting two other friends). When our friends arrived and sat down, we noticed that someone had left a jacket on one of the chairs at our table while we were sitting there (it wasn't there when we took the table).
We were a little confused but our friends sat down anyway. Right as the show was starting, an older guy came up and said that we were in "his" seat, as it had his jacket on it. He was pretty angry that we'd taken "his" seat, and we didn't feel like an argument, so we let him drag the chair off somewhere else, but it felt like he was being a bit of an asshole.
Is this a social thing in the US that I just don't get (I'm British) or was the guy just being an asshole?