I'm gonna disagree on this one. When you're just starting, the installation process can be nontrivial and the terminal window is scary. Having less friction there makes it easier to start and build some confidence. Once you have some idea of what you're doing, you can realize that you want different versions of the language.
I think this is a better argument for Apple providing a package manager with the operating system so that installing python is as easy as typing "brew install python3". Then it'll be just as easy for newbies to learn less standard languages.
If you have a tough time installing software that has maybe a dozen words or so of instructions in a clear easy-to-read format... you really have no business learning to code until you learn how to use your computer first.
Always ask for python2 or python3. "#!/usr/bin/env python" probably won't ever be able to detect and run both, so no matter what you wanted there will always be systems on which you won't get it.
If you follow a python 3 tutorial when your computer has python 2 installed on it, that's a terribly annoying way of fighting with your tools. I've seen it happen. I'd rather they don't risk it and just start fresh.
