A "Kindness" Gene?

According recent research, humans have a gene that determines their proclivity toward altruism. Here is report of the research, from ABC news. Here is the abstract (full article by subscription only):

Although 51 twin and adoption studies have been performed on the genetic architecture of antisocial behaviour, only four previous studies have examined a genetic contribution to pro-social behaviour. Earlier work by the author with the University of London Institute of Psychiatry Adult Twin Register found that genes contributed approximately half of the variance to measures of self-report altruism, empathy, nurturance and aggression, including acts of violence. The present study extends those results by using a 22-item Social Responsibility Questionnaire with 174 pairs of monozygotic twins and 148 pairs of dizygotic twins. Fortytwo per cent of the reliable variance was due to the twins’ genes, 23% to the twins’ common environment and the remainder to the twins’ non-shared environment.

Why does this "kindness" gene still exist? Wouldn't people who are free-riders tend to out-survive those with the "kindness" gene over time?

[thanks to BenS for the tip]