What evidence do you see that the low wage employees are angry about the circumstances in which they find themselves (excluding Ehrenreich)?

From the text:

Almost everyone is eager to talk, and I soon become a walking repository of complaints. No one gets paid overtime at Wal-Mart, I'm told, though there's often pressure to work it. Many feel the health insurance isn't worth paying for. There's a lot of frustration over schedules, especially in the case of the evangelical lady who can never get Sunday morning off, no matter how much she pleads.