Most Team fortress 2 requirement payment to have. Valve made team fortress 2 free, so it attracted more people, and more people means higher chance of someone buying an in-game item.
Also, It's been around since 2007, so a game that old has to be free these days. So, yeah.
Team Fortress 2 has been free to play for five years now and it's safe to say that going free to play didn't negatively affect the game. While some players (myself included) might have been annoyed to some degree that the money they original paid for the game had been reduced to a hat, it can't be denied that going free to play would have spiked the number of players and possibly increased the life span of game.