According to my understanding they have to fast to have their sins removed, so I agree that they are "working" for their "salary" of having their sins forgiven.

The Christian approach is different. Forgive the comparison but to me the Muslim faith looks more like a part time job than anything else. Christianity is about dedication, a lot like a marriage. It's something you're supposed to think about daily. To that end I agree with Jibb's view on fasting; that is, I see fasting as a way to spend quality time with God than working time.