Kinda makes sense, since this thing is basically a miniature computer. When iFixIt did their tear down, most of the inside of this thing is a heat sink. Could be that the surface temperature is high but, if that's just the heat bleeding directly off the CPU, 138F is pretty normal for a processor running at any appreciable speed.
My cell phone will get pretty hot on the back, too, if I play a game that pushes the processor for a while. Your PC/laptop doesn't do this because it has active cooling, so a fan blows the heat away with air, so it all goes out the vent. But even on a full-size PC, if you check the system health sensors, 138F (~58C) is a normal temp for a CPU with light load. Especially with no active cooling. The P4 on my media center (yeah, I know that's old) runs about 45-50C at idle.
TL/DR: The heat isn't really dangerous to the processor until it get up to about 70-75C.