Not that they were smoking in the workplace mind you, but merely that they were using a legal substance on their own time outside of the workplace.
As reported in the Lansing State Journal:
Okemos-based Weyco Inc.'s firing of four people who refused to take a test to determine whether they use tobacco has a lot of Michigan workers wondering whether they might be next.
This certainly raises a question as to how far employers may go to control their employees activities that take place on their own time. While it is legal for Weyco to have done what it did, it may prove to be going to far.