When you hire freelance writers, you expect them to work as a fully independent professionals. But when writers have to work as a temporary- employee of an agency, they lose the right to deduct their legitimate business expenses. And in many cases, the fees taken by the agency result in writers receiving less than fair market rate for their talent and experience, which results in dissatisfaction and discontent. For example, do not treat or supervise them as if they were employees, but rather treat them as independent vendors of services. Ask them to provide reasonable evidence of operating an independent business such as business cards and stationary, local business license, DBA notices, and so forth.
Some companies go so far as to require freelancers to be incorporated.