Since the subject line was a bit misleading, I have to wonder if all the facts are also here. I opened this thread assuming it had something to do with an EMPTY flight and it turns out it had to do with seat assignments.

If the flight was empty (or close to it since there were at least two passengers)... and the seat in question was not a special seat, I would have just plopped my butt into another seat. If this was a special seat, such as bulkhead or exit row, then a polite request might be in order. If the plane was at least half full, the request would be warranted as well.

In defense of the seat poacher, if the plane had only a handful of people and it was known prior to boarding, it would not be beyond MANY OF US to get on and just pick a seat. Though I do not make it a practice to grab a different seat, an empty plane is a different story. I have been on an almost empty plane where the GA said all rows can board and feel free to pick your own row.