1) Detached structures are not part of the dwelling. TR not required, nor is AFCI. Call it a 'loophole,' and you've got it backwards. Not too long ago, those things were the exception, not the norm
2) "Hospital grade" marking has absolutely nothing to do with what sort of wiring method is used. "Hospital Grade" simply means the device has met tighter plug retention and corrosion tests.
You want to unravel a conundrum, ask why you can't replace a "WR" device with a "hospital grade" device, when the hospital grade device meets even tougher corrosion tests than the WR one.