The "open unless posted" philosophy is just that, a belief or philosophy. Howver you are wrong in two aspects:

1) As a landowner you can close down your land in a heart beat, just post it.

2) You don't have to "legally" post it for it to be valid. Someone above pointed out the "legal posting" rules and that is an old belief and no longer true. If the land is private and the owner makes a "good faith" effort to either post, or ask people to leave, then it's closed, period. So if you have a spot where hunters or hikers or anyone else, is using to access it, a simple sign there is enough to have the land closed.

The only exception is public use tax status. If the land if filed under public use tax status, it HAS TO BE OPEN TO ALL.

Much of this is because there is much land, especially up north, that is private, yet owned by companies. If the law was flipped, then you'd need access from a corporation to go on the land.