Wouldn’t it be more accurate to say “the act of owning a big private ranch and trying to claim public land that it surrounds as inaccessible is effectively illegal?” The fact that the end result is that “big private ranches are illegal” doesn’t strike me as irrational. These people are well aware that the large swathes of land around them are public land. The fact that they may have purchased them under the assumption that no one would be able to access that land isn’t really an idea that garners a whole lot of sympathy.
It has been illegal since 1885, but apparently the state doesn’t want the feds involved (it’s a fed law), and the fed is trying to look the other way - most likely because of very rich donors.