Do you make a habit of interrupting people who are reading? If so I can just about guarantee that you're "that guy" to the people you're doing that to.
I don't think most people view a table for one at a cafe as a social setting with regard to total strangers. It will depend of course and there will be associated social cues; reading anything be it a screen, a book, or something else is a strong cue against unsolicited social interaction in almost any context.
It depends, it depends. You need to look at other signals. Are they extremely absorbed? Is it somewhere extremely quiet (like a library), or somewhere louder (like a coffee shop)?