The argument (for installing 2 as a default) is that a novice to the ecosystem will be disproportionately harmed by the inevitable Python3 experience:
Novice: "I want to do X"
Internet advice: "Use package Y"
Novice: "Okay <install, install>, wtf nothing is working"
<long frustrating debugging session>
Novice: "Oh, wow, this doesn't support Python3 yet. Now I have to ignore all the internet advice and forge my own path, OR port all my code back to 2.7! This language sucks!"
The alternative seems preferable:
Stuff works, but occasionally you don't get a whiz-bang feature (you "only" get 2.7's feature set, poor you). Five years pass, you learn the language. Now you've gotta learn a bunch of new habits, which sucks, but it's not as bad because now you're pretty good at Python, so the easy parts are easy.
I think they should be asking "How do I do X" before choosing a version to install. The answer might lead them towards IronPython or Jython, not a version of CPython from python.org. Some questions might even lead people straight to PyPy.
More and more questions have problems that can be solved by Python 3, but it's probably a pain in the ass if you start with 3 then work out how to solve your problem.
Novice: "I want to do X" Internet advice: "Use package Y" Novice: "Okay <install, install>, wtf nothing is working" <long frustrating debugging session> Novice: "Oh, wow, this doesn't support Python3 yet. Now I have to ignore all the internet advice and forge my own path, OR port all my code back to 2.7! This language sucks!"
The alternative seems preferable: Stuff works, but occasionally you don't get a whiz-bang feature (you "only" get 2.7's feature set, poor you). Five years pass, you learn the language. Now you've gotta learn a bunch of new habits, which sucks, but it's not as bad because now you're pretty good at Python, so the easy parts are easy.