I did a bit of googling - apparently, this is a big problem in Oregon. And it's all because of the real estate bubble, crash, and subsequent abandonment of so many houses by their owners. The banks foreclosed, homes were abandoned, and squatters moved in.
The laws in that state appear to be such that they must be legally evicted. The bank sells to an unsuspecting buyer, then says "Oh, squatters? That's your problem now." Laws here would not permit that.