Q:

Why blame the Australians?

I noticed the following comments under a hot meta post: Downvoting questions from new users

I agree this is horribly bad form. I blame the Australians. – Andrew
But @Andrew, as you yourself pointed out, there are no Australians here. – StoneyB
@StoneyB No, actually I said I never see them. They could be all around us, waiting in the trees like downvoting drop-bears,
  whispering, "Strewth, she's a beaut isn't she!" when they see their
  next victim. – Andrew

At first, I thought these comments to be rude and offensive, yet the upvotes suggest that they are not. I guess it's a well-known joke, but searching "blame the Australians" online didn't give me any useful information.
My question is: Why are Australians blamed?

A:

It's not a well known joke, nor are you missing any nuance of the English language. Presumably Andrew was alluding to an earlier meta thread of his - How come I never see any Aussies on here?, where he put a humorous slant on a genuine question. 
This would be known as an in-joke - a private joke that can only be understood by a limited group of people who have a special knowledge of something that is referred to in the joke.

A:

Specifically this joke is probably based on the fact that Australia is on the opposite hemisphere of where most English-speaking countries are, leading to a running joke of everything in Australia being upside-down. 
In this joke, the well-meaning Australian wants to upvote a post, but because they are "upside-down", they downvote it instead.

A:

What you are seeing is a common construct that ridicules the blame of unrelated groups by sarcastically laying blame when it's understood to be sarcasm. The poster is not seriously blaming Australians.
The 'Thanks, Obama' meme is a similar construct that is a response to the American right-wing blaming things on Obama that are difficult to link directly to him, such as the mortgage crisis. In response, the left-wing ridicules it by attaching 'Thanks, Obama' (in a sense: blaming Obama) for silly things like spilling your drink.

http://knowyourmeme.com/memes/thanks-obama
However, this isn't new and didn't start during Obama's presidency. One of the themes of the South Park movie released in 1999 is 'Blame Canada', where the residents of South Park refuse to see that they are the proximate cause of their children's use of bad language, and so they blame the country of Canada instead for a TV Show their kids watch.

Blame Canada song: https://www.youtube.com/watch?v=DBpgcZ1zYJs

