The reason that you can't charge back an employee is because you have a-lot more control over them. You can tell an employee how to do their job. Not to mention, they typically make a-lot less than a contractor, to charge back an employee is just oppressive.
A contractor can be charged back mainly because they can do the job however they want, as long as it meets all the specs, codes, etc. A contractor is also the equivalent as hiring another company, therefore the liability is on the contractor. With an employee the liability falls on the company, and not the individual.
Our employees don't see chargebacks, too many screw-ups and you're out of here. That is how it was meant to be according to the Fair Labor Standards Act.