Python was designed as a teaching language.
It's very easy to get started, and the Python ecosystem is very friendly to beginners. Just go check out archives for the Python Tutor list.
Replies like this one are extremely common - especially for non-help-vampires.
I have yet to find a community anywhere nearly as welcoming and friendly to newcomers. Plus a few years ago at PyCon they gave out Raspberry Pi's to all the attendees, so many Pythonistas will at least have a Pi kicking around even if it's just running XMBC ;)
And from a Pi-specific point of view, it's very very easy to start controlling the GPIO ports
import RPi.GPIO as GPIO
9 lines and you're done. All it takes to run this is
sudo python blink.py - there's very little that you have to understand about compiling or linking or anything else. You barely need to understand anything about programming.
Plus if you think about the fact that they have two options - recommend nothing, or recommend something - a minimal one-line recommendation for Python is probably the best one they could make. Anyone with a preference will probably gloss over it and go to their language of choice, and anyone with no experience will (hopefully) have the least frustrating experience possible.
: Though, I think Ruby is also pretty friendly to newcomers
: Even the difference between Python 2 and 3 isn't as painful as what I've heard
some of the problems are between Ruby versions.