alt Hacker NewsDepends on the layout. If its a large, sit-down restaurant with wide gaps between the tables, then yes it would be weird for me to go up to you and say "Hi, Stranger!". But at a coffee shop you might be sitting right next to me. We might even be sitting at the same table waiting for our food. Am I not allowed to talk to the person sitting right next to me? I ordered some food the other day and realized there were no free tables, so I asked a stranger if I could sit at his table and had a conversation with him and his buddy.
All of this is contextual and it doesn't take a screen or a book for someone to give off clear vibes of not wanting to chat. "Mind if I sit here" in a crowded shop is the expectation. Anything beyond that such as having a conversation with a total stranger depends on the subtle behavioral cues given off by the other party.
It's not my intention to be rude but based on your responses on this topic I'm guessing you're fairly oblivious to the relevant social cues. There's nothing wrong with that per se but adopting an attitude of "not my problem" is probably just going to aggravate the people around you.