Even if they aren’t doing it for free. It completely depends on the situation, and the pressure the employers apply to get people to accept. In an ideal wonderland, management would offer enough money so the workers find it worthwhile. In reality they often offer the bare minimum and apply as much hierarchical pressure as they can to get away with not offering anything at all. I’ve seen it countless times, and it goes something like: "You know I’ve been thinking about that promotion for you… You know what could really help? What about you give us some help next Wednesday night?"