From their website.
http://www.pokerstars.com/poker/tournaments/rules/
PokerStars uses a "forward moving button" rule in its tournaments. According to this rule, no player ever receives the button twice in a row; at the completion of every hand, the button is moved clockwise to the small blind. The implication of this rule is that when players are eliminated, there may be players who get a reprieve from one or both blinds. The forward moving button is common practice in online poker due to the technicalities involved in adjusting the blinds after players are eliminated. Since these are basically random occurrences, no player will receive any long-term advantage, and the rule is necessarily fair for all players.