Oh, my..you are right.
Apparently, squatters have rights in some states. Wow.
Well, Michigan is 15 years, NH is 20 years of continual occupation...and Alberta just abolished the "Squatters rights' altogether recently (and British Columbia and Ontario already did it beforehand).
I am not sure why US still has these somewhat ridiculous "rights" on the books; maybe so the available land is cultivated?
I believe that's what's referred to as "adverse possession". Even without that being in effect, squatters can be a pain to be rid of.