voting is free - why doesn't everyone do it?

The New York Times recently ran an article, "How the Other Fifth Lives," delving into the how the world's top 20% influences politics, amongst other things. 

In the graphic "People With More Money Vote More" there appears to be a positive correlation between income and voter turnout. At a glance, it would appear that the affluent are much more politically invested.

If so, does this have a downstream effect on bills that are introduced and legislation that is created?

The article's author, Thomas B. Edsall, quotes research: "Segregation of affluence...concentrates social capital and political power. Any self-interested investment the rich make...has little chance of 'spilling over' to benefit middle- and low-income families." 

There's nothing wrong with being rich. There is value in hard work, ingenuity and perseverance. However, there is a problem when eligible voters choose not to make their voices heard by not voting. 

What are your thoughts?