LFR has "exclusive" content just the same if you're looking for free loot, an exclusive lack of mechanics, bad players and a frustrating and unrewarding experience. That's something that can't be found in HM's
It's not "exclusive" in the sense that people "aren't allowed" to do it. People with lower skill or odd schedules choose not to do it. Just like most HM raiders choose not to do LFR. To call HM content "exclusive" because people choose not to do it is like calling LFR exclusive because people choose not to do it.
This "exclusion" is something the person puts on themselves, not the game.