restriction on sum: intentional bug?

[Alan G Isaac]

I expected this to be fixed in Python 3:

>>> sum(['ab','cd'],'')
Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
TypeError: sum() can't sum strings [use ''.join(seq) instead]

Of course it is not a good way to join strings,
but it should work, should it not?  Naturally,

>>> '' + 'ab' + 'cd'
'abcd'

Why doesn't duck typing apply to `sum`?

Thanks,
Alan Isaac