Don't Talk to Strangers (just stand up and eat)

Teachers were off today (provided they took classes over the summer), so I had my whole day planned out. Best Buy to use up some Reward Zone coupons, changing out Naji's cat litter, vacuuming (after my cleanup of Naji's litterbox), and then...the mall. I wanted to get a haircut before the quartet gig next weekend, and the musical the weekend after.
I got to the mall, and of course, it was crowded, but I didn't mind. I was just getting my hair cut (which usually takes all of 20 minutes). I signed in on the waiting list, and was told it would be about 40 minutes. Just enough time for me to grab something to eat for lunch.
As I waited in line to get my food, I was scoping out the seating situation, which looked extremely grim. As I took my tray, I contemplated asking to sit at a table with someone else, since many of the 4-seat tables only had one person at them. Luckily for me, one opened up, and I sat quickly and started eating.
When a man decided to stare me down expecting me to get up, instead of politely asking for my table (or even better, asking if he could just sit at one of the empty seats), I flipped. I asked, "Is there something I can help you with?" The guy lowered his head, shook it, and walked away, not even looking back in my direction.

So I ask: "Why is it not socially acceptable to sit in an empty seat in a public place?"...also, "Why do people resort to rudeness instead of opening their mouths, being friendly and asking if you're almost finished?"

Sighs...at least my hair looks good.