Yes, it would bother me if someone posted a property that they didn't own. Does the land owner know someone did this? If so, they must be giving them permission to do it. Same as a free lease I guess.
As a land owner, I expect everyone to ask me before they step foot on my land, even though it's not posted. They don't ask, they get thrown off and told not to come back.
If someone posted it without my permission, I would have them arrested.