They don't think. Some may be casual cyclists, with no idea of cycling etiquette. Others are just oblivious. It's like the people who go in the out door; no one ever told them to "stay right."
I would give them a polite heads up as to the problem and how to solve it. You might have to raise your voice to get their attention, but after that, I would be super polite and just give the facts, in as few words as possible. I despise riding on paths because of this (as well as walkers, dogs, and kids who aren't being supervised).
However, I experience this on group rides. About 5 years ago, MA passed a law that made it legal to ride 2 abreast if it is safe. Some are totally oblivious to this application of the law, and cause issues when cars are backed up behind them and other riders have to try to open a gap to let them in. When I lead rides, I tell people I want them to stay single file all of the time, except on very quiet rural roads, and then if they hear someone say "car back", they immediately make an effort to get over.