Back in September, Steam made some changes to how its user review system calculated an overall rating for each game. Now, Valve is making further changes based on how games were obtained by review writers. The calculations are still in progress, but from now on, only users who purchased a game will have their reviews count towards the overall score. If the game was played during an event such as a free weekend or received as a gift or for free through other means, it will not count.
Users who receive a Steam game for free can still write a review, but should not expect to influence the overall score. This change will not apply to free-to-play games, only games that require a monetary purchase upfront.
This seems like an odd decision. If anything, purchasing a game over receiving it for free is much more likely to cause a person to work to justify their purchase. Then again, Steam user reviews aren’t really a reliable barometer for a game’s quality anyway.
Source: Steam