Steinbeck was commenting on 'capitalist greed'. Clearly the banks were little concerned for the families that had lived and worked the farms that is evident by the bulldozers. The banks were only concerned with profit and how best to achieve it under the economic pressures of the depression and dust bowl. Steinbeck suggests the system 'failed' many in America and since the banks represented an economic arm of the federal government, it was the government that failed its citizens.
The Grapes of Wrath, is a great novel on the American culture and life back during one of the darkest times in US history: The Great Depression. As you mentioned in your question, the Economic System was certainly responsible for helping us get out of the Depression, but it is also what contributed to getting us there.
Throughout the novel, readers get insight into the Joad family and their struggles as they make their way across Route 66 to get to California. The US economic system throughout this book, mentions a great deal on the suffering of the lower class, the inability to find work (aka high unemployment), and the inequality of the higher and lower classes in society. The Joad family is a great example that details this economic system.
As the family travels across the country, we get witness into not only the suffering of the family, but also the people they run into throughout the book. The lower class is always suffering, while the higher class who owns property rights and the means to all the capital, are striving. Unfortunately, this inequality was inescapable in the majority of the Great Depression. Since unemployment was so high, people were desperate to take any possible job that came there way. Before minimum wage laws, this lead to a very low wage for these type of workers simply because they had no other option.
The inequality that the economic system caused in The Great Depression, is well seen throughout this whole novel. I hope my explanation helps you a little bit with your question.