The behavior is according to the specification:

  http://speleotrove.com/decimal/decarith.html


The idea behind it is that 1/0 can be reasonably defined as infinity,
whereas 0/0 is undefined.  You can see that if you disable the exceptions:

>>> c = getcontext()
>>> c.traps[DivisionByZero] = False
>>> c.traps[InvalidOperation] = False
>>> 
>>> Decimal(1) / 0
Decimal('Infinity')
>>> Decimal(0) / 0
Decimal('NaN')
>>>