You either have to trust Docker (a fairly well known project built by reputable people) isn't going to root your machine, or download the source yourself and audit it.
This is no worse than suggesting you "git clone whatever; cd whatever; make" (aside from the lack of SSL)
...and everybody else on my network, with that method. Doing that I don't even get the chance to think "Hey wait a second, why was this only 50 bytes of shell script...".
The reason that you see outrage for this "method" is because it is born of laziness and far too reminiscent of more disturbing times in computer security.
Why do we need an instruction on downloading the source to begin with? It really just promotes bad habits with those who know no better, i.e. new/inexperienced developers. The problem is when people see instructions like that on 20% of the guides they read in earnest, trusting that everything is OK if enough people say it. One hopes they stumble upon a discussion like this so that they can consider the consequences but that just isn't going to happen to everyone. True, one should exercise equal caution while cloning, gem-ing[1], etc. It would be great if authors would just link to the source and paste the relevant lines from the README if necessary.
[1] http://andre.arko.net/2013/03/29/rubygems-openssl-and-you/
