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I've investigated a bit more.
First, I've tried with Python 3.7.0a1 . As you'd expect, PEP 537 means
this behaviour now also occurs when no locale environment variables at
all are set.
Second, I've looked through locale.py a bit. I believe what it calls the
"aliasing engine" is applied for:
 - getlocale()
 - getdefaultlocale()
 - setlocale() when passed a tuple, but not when passed a string
This leads to some rather odd results.
With 3.7.0a1 and no locale environment variables:
 >>> import locale
 >>> locale.getlocale()
 ('en_US', 'UTF-8')
 # getlocale() is lying: the effective locale is really C.UTF-8
 >>> sorted("abcABC", key=locale.strxfrm)
 ['A', 'B', 'C', 'a', 'b', 'c']
Third, I've checked on a system which does have en_US.UTF-8 installed,
and (as you'd expect) instead of crashing it gives wrong results:
 >>> import locale
 >>> locale.setlocale(locale.LC_ALL, ('C', 'UTF-8'))
 'en_US.UTF-8'
 >>> locale.getlocale()
 ('en_US', 'UTF-8')
 # now getlocale() is telling the truth, and the user isn't getting the
 # collation they requested
 >>> sorted("abcABC", key=locale.strxfrm)
 ['a', 'A', 'b', 'B', 'c', 'C']