Null may be a valid value for an element of a list.  Say your list contains elements which represent some optional data about a list of users and is stored in the same order as the users.  If the extra data is populated then your list will contain the additional data otherwise the slot corresponding to a user will be null.  (I'm sure there are better examples, but you get the idea)

if you don't want to all nulls to be added then you could wrap the array list with your own wrapper which threw when null was added.