@joshbeaudette - I disagree that crony capitalism is separable from free market capitalism. More to the point free market capitalism is only a thought experiment. Something that can only exist in theory. It will ALWAYS devolve into crony capitalism. And even totally free market capitalism would have the same troubles. But lets not go into that here.
The point I really want to address is the question of why it is that someone being rich causes other people to be poor. The answer to this question comes from Marx (*gasp* The horror! The horror!). According to Marx, industrial capitalism (and this argument also applies to post-industrial capitalism) is based on people selling their labour to capitalists, or as Marx called them, the bourgeoisie. The thing is, value is created when people work to create a product or service that can be sold to customers. The problem here is that the capitalists purchase your labour for less than it is worth to them. So for example, lets say you work in a factory making refrigerators for $12 an hour. And in an hour you make 1/10th each of 10 refrigerators. Basically it means you've made equivalent value to 1 refrigerator in that time. Now the capitalists turn around and sell those fridges for $2000, making a huge profit in the process.
Now, you the worker need a refridgerator. You go to the company you work for to buy one, you are then paying your boss, with the money you earned making the fridge, at 166.67 times what he paid you to make it. (Now obviously I'm ignoring the costs of the material here as well, and those do need to be taken into consideration, but believe me when I say there is no material cost going into building a fridge that justifies it's retail markup). Now, that is how capitalism forces people to distribute wealth upwards however, due to concepts like generalized reciprocity obviously for this to take place you don't need to purchase a fridge, if you earned money making one. the result is the same whether you give your money to your own boss or someone else's.
The point is, wealth is accumulated by capitalists because they appropriate it from other people.