Bottom line from a person that spends alot of time out there and who has alot of maps. Its not always easy to tell where state boundaries are, and where private ownership is, where timber company land is, and where farmland is. Sometimes its obvious, sometimes it is not. If I see a sign, then I know better or have to take extra effort.
There are lots of games played these days with people posting stateland, people taking down stateland signs etc. If someone walks past a no trespassing sign, they don't have a lot of excuses.