It doesn't throw a ConcurrentModificationException because, as vandale said, the iterator only checks for comodification on next(). Here's a portion of the Iterator instance returned by an ArrayList:
public boolean hasNext() {
return cursor != size;
}
@SuppressWarnings("unchecked")
public E next() {
checkForComodification();
int i = cursor;
if (i >= size)
throw new NoSuchElementException();
Object[] elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
}
hasNext() simply looks to see if the cursor is pointing at the last index of the list. It doesn't check to see if the list was modified. Because of this you don't get an ConcurrentModificationException, it just stops iterating.
