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Yuck, I agree that that's pretty icky.  But the disassembly makes things 
clear:

>>> dis.dis(compile('t[0] |= a', '', 'single'))
  1           0 LOAD_NAME                0 (t)
              3 LOAD_CONST               0 (0)
              6 DUP_TOPX                 2
              9 BINARY_SUBSCR       
             10 LOAD_NAME                1 (a)
             13 INPLACE_OR          
             14 ROT_THREE           
             15 STORE_SUBSCR        
             16 LOAD_CONST               1 (None)
             19 RETURN_VALUE        

In fact...

>>> s = set([1])
>>> t = (s,)
>>> t[0] |= set([2])
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: object does not support item assignment
>>> s
set([1, 2])
>>> 

Oof.

Not sure what to do about this.