http://blumenthals.com/blog/2012/01/31/is-google-intentional...
They recently got rapped by the FTC for it.
http://wallstcheatsheet.com/stocks/ftc-googles-ad-practice-i...
The difference is that the negative Microsoft news tends to float on top of sites like HN more than positive news, and the reverse is true for Google so this alters perceptions of people.