In the sisters' opinion, Jim Crow laws are created because black people have started to "get their piece of the pie" and "accumulate some wealth" (4.10.2).
Jim Crow Laws are especially opposed to the mixing of the races, despite the fact that many people in the South (whether "white" or "black") are of mixed racial descent.
The sisters encounter the new laws for the first time when they're forced to the back of a public trolley. Then, when they exit the bus, they notice a segregated water fountain! Bessie, being one bad mother shut-your-mouth, drinks out of the white water fountain out of spite. We <3 Bessie.
A few days later they're kicked out of their usual drugstore because of their race. The irony is that the man who kicks them out also happens to have "a colored family on the side" (4.10.10).
These laws set back race relation a long way. It's around this time that Henry starts only buying from black-owned businesses to support his people.