The problem seems to usual surround who has legal possession of the property.Why wouldn't squatters be immediately arrested for possession of stolen property then if they have a legal lease they can prove their innocence and go after the accuser for wrongful accusations?
Possession of a stolen stereo was good enough to get arrested for possession of stolen property back in the day…
To play devil's advocate:
Suppose you do have an agreement to rent/lease a property and the owner decides for some reason he wants you out now. He accuses you of squatting but you are legally there. How does a police officer decide who's in the right? You are proposing someone who is innocent be thrown out, have to hire a lawyer, maybe lose work, money, and time to prove he was in the right in the first place?