The distinction between mutable and immutable is important, but more important here is the difference between

    >>> dict['a']=3

and

    >>> dict['a'].append(3)

The first rebinds dict['a'] (and in the process mutates the object dict is bound to).

The second mutates the object dict['a'] is bound to (and in the process binds dict['a'][1] to 3 [EDIT: not really -- 1 is not a name and so doesn't have a binding]).

I think looking at it purely in terms of mutable versus immutable makes it more likely for a newbie to expect the assignment above to work differently depending on the mutability of dict['a']'s old or new value. (I have seen this type of error.)

EDIT 2: I think a better way of making my point is that this:

    > When you reassign one of the dictionary values
    > pointing to an existing string, it can't change that
    > string, so it creates a new one.

can be taken (incorrectly) to imply that reassigning a dictionary value pointing to an existing list will mutate that list.