“One would think the squatters could be charged with trespassing.”
Except when it comes to property, the reason they squat is knowing full well it will take awhile to remove them. Meantime, they live rent free.
http://en.wikipedia.org/wiki/Possession_is_nine-tenths_of_the_law
“Possession is nine-tenths of the law”
The solution is if they are found to be maliciously squatting to send them to prison for a long, long time.