A few years ago I experienced this for the 1st time in NYC. I went to my favorite South Indian buffet (Chennai Garden) which granted is literally a hole in the wall. Also, due to the cheap prices and lunch rush it was very crowded. I was by myself. I have no problem waiting for a table. I was given a seat and began to dig in when 10 minutes later another single diner was plopped down at my table (I was not asked). I didn't want to be rude to this diner because he was already laden down with food and there quite frankly was nowhere else for him to go. However I was extremely perturbed and felt that my delicious lunch was ruined.
I realize that this is mainly a personal/cultural issue. Some people have no problem with a lack of personal space. I also know that many cultures (especially in Asia) are okay with it; sometimes even prefer it. However, this restaurant was in NYC where our culture generally feels the opposite (for better or for worse). If it's a restaurant's policy I wish they would let you know upfront or at least ask you. For ex, this weekend I was at a vietnamese place in Chinatown. It was very busy. I was asked several times if I wanted to share; each time I said politely "no thank you. I do not mind waiting". While they were nice about it I was still a bit paranoid and made sure to put my coat on the other seat!
How do you feel about being forced (or even asked) to share a table?