A quick look at the childminder board would answer your question, I think.
Anyway, to summarise the points that have so often been made:
A childminder has her own contract. It's up to her whether or not she charges for holidays and sickness. If you sign the contracy, you're agreeing to this. You're therefore being unreasonable to complain about terms that you have agreed to.
A childminder's sickness is often because of the bugs a minded child has brought it. Perhaps if parents didn't dose their children up and try to pass off diarrhoea as "teething", childminders wouldn't feel justified in charging when they take those rare days off sick. Most childminders will try to keep going as they know that parents rely on them. Sick days aren't taken lightly.
Same with holidays. Childminders don't just down tools and relax on their holidays. I spend a lot of the time in preparation, notes, learning journeys, rearranging the house, giving it a thorough clean because the mindees make lots of mess, draw on walls, what have you. Even though your childminder may not physically have your child with you, he or she will probably still be working for the benefit of your child.
Not all childminders charge for sickness and holiday. Those that don't invariably factor in sickness and holiday into their fees, as do many self-employed people. They'll have a higher rate, therefore (though I bet it's nothing like the high rates that other self-employed professions charge). Perhaps those childminders that charge for holidays and sickness are just being transparent in their charges and letting you know exactly what you're paying for.
I think that's everything? If you're wondering, I don't charge for either sickness or holiday: I factor it in. I also get people whinging that I charge more than some of the other childminders in the area. Can't win really, can we?
I wonder if we'll get that other chestnut about why is a childminder on the internet when she should be working? ;)
To answer that one to those who are worrying about my charges' safety and well-being, I work with another childminder. Lucky me. Most don't get a break at all. The computer is in the playroom with the children and I can see them at all times. I hope this addresses any concerns you may have :)