This happens far too often and needs to be stopped.
If you walk into a restaurant and there are only one or two other diners (singles or couples), do not ever sit at the table next to them, or at a booth which shares a back with them. It's unspeakably rude. If the waitress tries to seat you there, tell her you want another table. If they say "seat yourself", put yourself an equal distance between whoever is in the place. Never, under any circumstances, seat yourself at a table next to, or a booth sharing a back with, another diner -- single or couple.
Thank you.