From riding on buses and trains I've learned that many people have no qualms about conducting even rather private phone conversations loud enough to make it impossible not to overhear them. For that reason it would seem to me that people wouldn't mind someone glancing over their shoulder to view what they're doing on their phones. And frankly, either they don't, or I intrude considerably less than I think I do. Recently someone I was sitting next to on a bus offered me his newspaper. I think that he'd finished reading it, but he may also have been distinctly aware that I'd been glancing at the headlines as he turned the pages. Nothing of that sort has (yet?) happened when I've glanced at people's phones, but on the whole I haven't been as close to them as I was to the person with the newspaper on that bus.