You can't just choose not to work in the UK. If you resign from a job without good reason, you can have benefits stopped for 6 months. You probably can't survive for 6 months without food. In practice you might be able to get reduced some form of benefits but I don't think it's guaranteed. The term "slavery" might be hyperbole (you are paid, after all) but you might not have much choice about working.
The article implies it's easy to get fired, so no need to resign...
Hyperbole much? Note that the article is talking about the UK, a country with very extensive (and expensive) social benefits system - so if these workers didn't want to work, they could simply choose not to, and still survive just fine. That's in no way comparable to slavery.