Nevertheless, that is the behavior if NotImplemented is returned by a method.  Here's some code to demonstrate:

--8<---------------------------------------
from collections import Counter

class Spam(int):
    "for sake of example"
    def __radd__(self, other):
        other[self] = other[self] + 1
        return other

s = Spam(5)
c = Counter()
print(c)
c += s
print(c)
c += 9
--8<---------------------------------------

before the patch
-------------------------------------------
Counter()
Traceback (most recent call last):
  File "blah.py", line 13, in <module>
    c += s
  File "/home/ethan/source/python/issue20284/Lib/collections/__init__.py", line 738, in __iadd__
    for elem, count in other.items():
AttributeError: 'Spam' object has no attribute 'items'
-------------------------------------------

after the patch
-------------------------------------------
Counter()
Counter({5: 1})
Traceback (most recent call last):
  File "blah.py", line 13, in <module>
    c += 9
TypeError: unsupported operand type(s) for +=: 'Counter' and 'int'
-------------------------------------------

As you can see, we get better support for other objects that know how to add themselves to the container in question, and a nicer and more correct error message for objects that do not.

As I said earlier, the only decision we should have to make here is whether to check for a Counter, or just something with a .items attribute, but either way we should be returning NotImplemented.