While a single user is unlikely to be sued, software piracy is illegal under the Digital Millennium Copyright Act (DMCA) and similar international laws. More importantly, using illegal software in a professional studio environment invalidates your business insurance and opens you to liability.
If you are a hobbyist, the price of the product key might be hard to justify—you can often get by with cheaper third-party apps or the free software Nikon offers with their cameras (NX Tether). Nikon Camera Control Pro 2 Product Key