Google’s recent announcement to give a (currently still small) ranking boost to websites using HTTPS is undoubtedly going to make the web safer for everyone.
Missing from the discussion is the fact that unencrypted content and unscrupulous ISPs present a small, but growing threat to Google’s business model. Google depends on ad revenue, and insecure connections allow third parties to tamper with data while in transit. ISPs can use this to their advantage by injecting their own ads. This is already happening: see here or here and this topic on reddit. It was also briefly mentioned in one of the comments under the original “ranking boost” announcement:
HTTPS ensures data integrity and would make ad injection not only technically far more difficult, but also most certainly illegal. This also explains why Google says that even simple “content sites” should use HTTPS: they might not collect any user data, but they can still serve ads.
“Being a good citizen of the web” and making the web safer for everyone sounds nice and is certainly something many people working at Google have in mind. However, it would be naive to assume that Google isn’t also looking out for it’s own commercial interests.