Web developers who intentionally leave backdoors in their code to ensure their work is not used without payment may face serious ethical and legal consequences. Although it may seem like a viable solution, incorporating a backdoor comes with potential security risks and legal violations, and it can even lead to fines and jail time.

According to the Computer Fraud and Abuse Act of 1986, accessing computer systems in a way that uses higher privileges or access levels than permitted is illegal. Intentionally damaging the system or data is also a crime, and the penalty for violating the CFAA can include sentences of up to life in prison and large financial penalties. Even accessing files through an unapproved manner could result in fines and up to five years in prison.

In addition to legal consequences, leaving backdoors in code can damage the reputation of the developer and negatively impact future profits. It can also add a potential vulnerability to the client's website, exposing it to potential attackers.

Instead of adding backdoors, developers should set clear expectations with clients regarding deliverables and how they will be impacted by late or missing payments. Written contracts should be used to specify these expectations, and a development server under the developer's control should always be used. Once agreements have been met, the code can be moved from the development server to the client’s production server.

In conclusion, developers need to consider the potential ethical and legal risks of leaving backdoors in their code. It's always better to use secure coding practices and clear communication with clients to ensure everyone's needs are met without violating the law or compromising security.