You are correct that in our current capitalist system, the employees were in the wrong.
The problem is the capitalist system itself is wrong.
The Antiwork community is here to as a counter culture to the capitalist/corporate culture that has become so ingrained in our society at large. The idea that a person is forced to stay at work for their entire shift, even during times where there is no work to be done, is a problem. That is essentially where the term "wage slave" comes from, because while we are on the clock we lose our freedom to do what we want with our time.