I think that Steinbeck did a very admirable job in raising questions about capitalism and its presence in American Society. He understood that removing it and overthrowing it would be naive. Rather, he sought to give voice to those who suffer under it. In works like "Of Mice and Men" and "The Grapes of Wrath," one does not leave with the feeling that capitalism needs to be abolished, but rather it has to account for all individuals in its path. There has to be some level of understanding and economic and political redress for those who are trapped underneath it, pinned under its weight. He seeks to bring a voice to those who are silenced in the capitalist system. In the process, he is able to bring forth a strong criticism of capitalism in the hopes of making it more accountable for all individuals.
Stopping capitalism is like trying to stop a glacier. Steinbeck was not naive enough to think he could stop it. But he wanted to stop the exploitation of the "little guy": farmers, migrant workers, and local "mom and pop shops." So, he denounced capitalism's predatory practices as a means of social protest in his fiction.
Steinbeck saw his idyllic Salinas River Valley in California overrun with corporations during his writing career. Like the pragmatism (scientific determinism) of which he infuses in his fiction, Steinbeck knew that man was a victim of socio-economic forces. Though I wouldn't call him explictly socialist, there is certainly an undercurrent of anti-capitalism in his Dust Bowl fiction.
Steinbeck's populist writings about the Dust Bowl Oakies and migrant farmers did show the exploitation of the working class without unions and labor laws. The Joads and Lennie and George, in particular, had little protection under the current laissez faire system of hiring and firing practices.
In The Grapes of Wrath, bosses would send out handbills advertising jobs so they could pull the "bait and switch": send out 2000 handbills; attract 500 families for 100 jobs--all to drive down the cost of labor.
It wasn't until the Joads reached the government camp where they were protected and represented. This is Steinbeck's haven: strong federal government-run programs, where families have rights, a seat on the council, protection from bullying. He was a strong advocate of union-organized labor.
So says Enotes:
Upon taking office in 1933, President Franklin D. Roosevelt launched a comprehensive agenda of government programs to combat the Depression. Collectively called the New Deal, these programs included new federal agencies designed to create employment opportunities and to improve the lot of workers and the unemployed. Among the many such agencies, the one that most directly touched the Okies’ lives was the Farm Security Administration (FSA). Operating under the authority of the Department of Agriculture, in 1936 the FSA began building camps in California in which the homeless migrants could live. Ten such camps were finished by the following year. Steinbeck visited several in his research for The Grapes of Wrath. He had the Joads stay at one—the Arvin Sanitary Camp, also called the Weedpatch Camp, in Kern County. The intention was that the orchard owners would follow this example and build larger, better shelters for their migrant workers. This never came about, however, and many families ended up staying at the uncomfortable federal camps for years.
In an attempt to defend their right to earn living wages, migrant workers tried to organize labor unions. Naturally, this was strongly discouraged by the growers, who had the support of the police, who often used brute force. In Kern County in 1938, for example, a mob led by a local sheriff burned down an Okie camp that had become a center for union activity.