It's like when you're teaching and the students don't understand a thing you said, they'll have no questions. It's always the kids who understood 75% of what you said who has a question. One of the goals of every liberation movement (labor, feminism, racial justice, etc.) is making people aware of their shared struggle and it's causes. The Teamsters are far from radical but the fact that things can even be negotiated at all has a way of making people aware that decisions of management are not forces of nature.
If you were making a low wage and easily replaceable would you bitch about your job to random people who asked you if you liked it or smile and say its great?
If the warehouse workers were happy with the status quo, why would they be trying to unionize in the first place?