I strongly disagree. Even if we ignore confidentiality and security issues, there is a fundamental difference between contractors and employees. Employees are generally paid less than contractors and are provided stability inn return the company gets a stable employee.
Contractors are expected to be shorter term and change tasking if new opportunities arise, even for a short time.
If both parties enter into a deal then it's fine, if one is misrepresenting themselves it's not.
With companies it's more transparent and the contractors are bound to confidentiality and other codes of conduct. If something goes wrong the issue can be traced and addressed.
When an employee does it without informing the employer or signing an agreement that holds the contractor to the same standards as the employee the chain of trust, transparency and accountability is broken.
Contractors are expected to be shorter term and change tasking if new opportunities arise, even for a short time.
If both parties enter into a deal then it's fine, if one is misrepresenting themselves it's not.