The reason join() isn't a method on lists is that it would mean that every class that wants to act like a list would have to implement it (or inherit from someone who does).
If you write your own class that is iterable but doesn't inherit from list, it would not have a join() method unless you wrote one. But it works as an argument to '\n'.join() for free since that can operate on anything iterable.
The other problem with sticking methods like join(), sum() and any() onto collections is that whenever a new method is added (for example, any() was added in python 2.5), it would cause confusion for anyone who has a list-like class that happens to have a method with that name that does something different.
If you write your own class that is iterable but doesn't inherit from list, it would not have a join() method unless you wrote one. But it works as an argument to '\n'.join() for free since that can operate on anything iterable.
The other problem with sticking methods like join(), sum() and any() onto collections is that whenever a new method is added (for example, any() was added in python 2.5), it would cause confusion for anyone who has a list-like class that happens to have a method with that name that does something different.