By virtue of the fact that Tom Joad is on parole and cannot leave the state of Oklahoma, the Joads are constantly aware of the law and of trying to adhere to the law. Much of The Grapes of Wrath deals with families who are treated poorly by the law, whether in their hometown or in their California towns. We begin to question the validity and the value of the law as we watch the Joad family, and many other families like them, fight tooth and nail to survive.
The Law prevents survival in the world of The Grapes of Wrath.
Without self-imposed laws, the migrant families would not be able to co-exist or survive.