What is Voluntary Compliance?

Legal Definition
Voluntary compliance is one of possible ways of practicing corporate social responsibility.

Voluntary compliance is seen as an alternative to the state-imposed regulations on a company's behavior. Proponents of voluntary compliance argue that it is in a company's own interest to behave socially responsibly and that in pursuit of good public image, the company will withdraw from doing actions which could damage its perception by the public. Thus there is no need for state regulations.

On the other hand, opponents deem that companies may claim to voluntarily adhere to self-imposed regulations but in practice they often follow profit maximizing behavior, often violating stakeholders' interests. However, such behavior may be problematic not only morally or ethically but also legally: corporate codes of conduct may give rise to legal obligations pursuant to national laws of European Union member states implementing the Unfair Commercial Practices Directive or pursuant to consumer protection laws in other jurisdictions, including (subject to the effect of the Federal and State constitutions) the States of the United States of America.

In the United States, voluntary compliance may also refer to an argument made by tax protesters, who suggest that payment of income tax is voluntary, and not legally enforcable. However, this argument is rejected by the Internal Revenue Service, and has not been accepted by the U.S. courts.
-- Wikipedia
Legal Definition
Assuming that tax payers will comply with all tax laws.