If there are no available tables because there is one person camped out at each table with their stuff spread out, I have no problem walking over to a table and taking a seat and putting my coffee down on whatever space I can find -- or create.

I don't ask "Is this seat taken?"

Clearly it isn't, so I just sit down.

If somebody is sitting on their own at a table with at least one more seat, other customers are entitled to sit down in that other seat.

Nobody has ever protested when I've done this by the way. They usually just clear a little space for me on the table and keep working.