Before you get too excited about "allemansretten", or "every man's right", keep in mind that there are strict limits and responsibilities that go with it. Typically a person is not allowed to camp in the same place more than one night, must stay away from built up or developed areas, cannot build a fire, and must not harm the land in any way.
From what I have observed of homeless camps, such restrictions are ignored - which is in fact why it bothers people living in the area.
Besides this look at the article you linked to and its notes on Northern Ireland, which it notes has the most severe restrictions on this sort of land use - which is to say it doesn't allow it at all. The people inhabiting Northern Ireland are closely related to one of the biggest ethnic groups in the United States, whose ancestors immigrated from there in the 1700s. Before that their ancestors spent centuries in the territory between England and Scotland, which was an area where there was constant warfare. As you might imagine, in such circumstances people get rather protective of their territory. Keep in mind that they brought these attitudes with them to America.