You may be assuming that no consideration is given (consideration being necessary to create a legal contract) - but in general charities and public goods are th

You may be assuming that no consideration is given (consideration being necessary to create a legal contract) - but in general charities and public goods are the exception to this; the public good done is assumed to be consideration according to law. Therefore, for example, pledges to charities are regarded as valid contracts that can be enforced.