Many companies have extensive policies and employee handbooks but very little for contractors. Most contractors are familiar with the NDA and contract they sign that tells them about the basic policies areound security, behavior, etc expected from them but very few discuss how to handle discrimination or harrassment from the hiring company's employees. These incidents could be anything from a manager being unreasonable about expectations, being rude or talking harshly, or in more serious cases being abusive or flirtatious knowing the contractor doesn't have mcuh power in the relationship because the hiring manager could cook up any reason to fire the worker. Other problems arise from social media polcieis where contractors may complain or do things that may reflect poorly on the company and its not clear what they can or cannot do.
As times have changed and these incidents occur increasingly, it is time for companies to start adding these policies to their contracts with the contractors and temps. At the very least the company needs to identify who the contractor is expected to go to in case of a problem- is it the staffing agency or the HR department of the company or some other person in Procurement.
Ideally it should be the HR Manager of the hiring company as they are the only on with any oversight responsibility of the hiring manager. The staffing agency's incentive is to make the problem go away and most agencies wouldn't want to escalate an issue. Procurement is not geared to handling HR issues and will only redirect to HR or the staffing agency. However it seems HR of hiring companies are usually stretched handling fulltime employees and not geared for contractors- so it would require a policy and job duties change before this would happen.