Well, this is "Python for Beginners", so that name will attract the kind of person who has never touched the command line. Likely they'll be using a system in which Python is already installed, but they still won't know how to access it.
Another thing to keep in mind: this kind of beginner will also have trouble finding these "standard" tutorials that have up-to-date info. No, it's not that they can't use Google, but they don't have enough technical sense to tell what's a good source of this installation info from what's a bad source. Hence, it's good to have this super-basic info packaged into any kind of tutorial that purports to be for "beginners".
True. A search for python tutorial returns the official python tutorial. But, I hear your point. It should be added, or a least a link to the very beginner material should be provided.