Anti-strike laws: the new slavery

Today is a national day of protest against the Australian government industrial relations "reforms". For those not up-to-date, they're taking away a huge swathe of rights from Australian workers including nationally-agreed industry award rates, the right to appeal against unfair dismissal if you work for an employer with less than 100 employees, and remove penalty rates for Sundays, public holidays and unsociable hours.

Government departments have warned their staff that they could face fines and other penalties if they attend.

Here's the thing I don't get about these anti-strike laws: how can it be illegal to withdraw your labour? Isn't that just slavery under another name?