The decision to use SSL as one of the signals for scoring (not the only signal, but one among many) is based on the idea that linkfarms, Web scrapers, and black-hat SEO sites tend to be transient and don't employ SSL. A site that employs SSL is statistically likely to be a higher-quality site.
I have switched many of my sites to SSL, though I've discovered in the process that some ad networks are, unbelievably in this day and age, still not able to serve ads securely--meaning some browsers won't display ads on SSL sites.
