While both Zanuck and Ford were considered politically conservative, Zanuck was definitely concerned that the film would be seen as pro-Communist. He sent investigators to the Dust Bowl who reported the plight of the Okies as valid. How he came to endorse Johnsons pro-Marxist script is a task for a spiritualist, not me. Tom Joads big plea for social justice at the end would fit right into a MoveOn rally today. Roger Ebert speculated that Americas alliance with the USSR during WWII probably dulled any Com-Symp criticism of the film at the time. But times have changed.
Some literary critics have actually complained that the film dulled the politics of the novel. The final scene of the novel would never have made it onscreen...not then anyway.