According to the Python 2.7.2 documentation:
An object is hashable if it has a hash value which never changes during its lifetime (it needs a hash() method), and can be compared to other objects (it needs an eq() or cmp() method). Hashable objects which compare equal must have the same hash value.
Hashability makes an object usable as a dictionary key and a set member, because these data structures use the hash value internally.
All of Python’s immutable built-in objects are hashable, while no mutable containers (such as lists or dictionaries) are. Objects which are instances of user-defined classes are hashable by default; they all compare unequal, and their hash value is their id().
A tuple is immutable in the sense that you cannot add, remove or replace its elements, but the elements themselves may be mutable. List's hash value depends on the hash values of its elements, and so it changes when you change the elements.
Using id's for list hashes would imply that all lists compare differently, which would be surprising and inconvenient.