Python Strangeness

I'm one of the teaching assistants (TA) for 6.00 this semester.  Another TA sent this to our mailing list:

This is just for your amusement. I came across an interesting quirk of python.

First the normal stuff:

>>> [1,2,3] + [4]
[1,2,3,4]
>>> (1,2,3) + (4,)
(1,2,3,4)
>>> (1,2,3) + [4]
Traceback (most recent call last):
 File "", line 1, in 
  (1,2,3) + [4]
TypeError: can only concatenate tuple (not "list") to tuple

>>> [1,2,3] + (4,)
Traceback (most recent call last):
 File "", line 1, in 
  [1,2,3] + (4,)
TypeError: can only concatenate list (not "tuple") to list

>>> l = [1,2,3]
>>> l + (4,)
Traceback (most recent call last):
 File "", line 1, in 
  l + (4,)
TypeError: can only concatenate list (not "tuple") to list

So far so good, but now:

>>> l += (4,)
>>> l
[1, 2, 3, 4]

But it's not symmetric:

>>> l = (1,2,3)
>>> l += [4]
Traceback (most recent call last):
 File "", line 1, in 
  l += [4]
TypeError: can only concatenate tuple (not "list") to tuple

Which is to say, list's __iadd__ function is polymorphic to tuples, but
tuple's isn't to lists, and list's __add__ isn't either. Odd and a little
ugly, but yeah. Anyone know whether this is a bug or a feature?