Definitely both sides have correct points here. I personally would never turn someone in because they might be over a fortnight at one spot. However, come day 12-13, I'll be moving on to another location.
As for the above real estate takeover, here in Texas, it is called adverse possession. If one doesn't keep their land surveyed and someone encroaches, if something isn't done within 5 years, it is theirs. Plus, if the people use a road on your property, they can claim it as an easement as well.
People will try to do this too. If one doesn't either get the other party to sign some form of lease/rent/contract (or at least acknowledge that they have your permission), they can take your property, and even your livestock.