I have hiked a few sections of the Appalachian Trail (AT) in the Smokies over the years.

The "3-sided shelter" is not a tarp or any other kind of portable shelter. It is a small roofed building with 3 solid walls, a corrugated metal or shingled roof, and a chain link fence "front" to keep the larger critters out. Many sections of the AT have these shelters sprinkled along them such that many of the AT hikers don't carry tents or tarps at all. It is a very common practice. That's not to say that this group of 4 didn't screw up.

The AT shelters merely keep the rain off of you, as long as you don't stay under a leaky part of the roof. The wind sails right through them. They often have a stone "hearth" where you can build a fire.