I think that Steinbeck shows in Chapter 20 that the real value to arresting a migrant is present if they wish to organize. One of the harshest realities that Steinbeck shows in Chapter 20 is how the workers are poised to organize. They are shown to be tired and frustrated with being cheated and denied their opportunity to increase their lot in life. The demand for fairness translates into organizing and forming a coalition against the unfairness of the bosses, who have already manipulated the "law" on their side and use it to break up any hopes of worker empowerment. Floyd Knowles' questioning of the boss is one such example. The fact that owners and those who profited from migrant labor were not "on the level" is reflected in this exchange.
The profit for arresting a migrant worker who sought to unionize or collectivize is extremely worthwhile to those in the position of economic power. Steinbeck shows that there is a profit for the bosses in stopping the theorizing of worker empowerment. With unionizing, the demands for bosses to be "on the level" would cut into profits and cut into power. In this, there is value for the bosses to stop this by arresting migrants who speak of such an issue, intimidating other workers to be silent and submissive to an unfair system that is "tryin' to break" the workers.