Contracts must have 'legality of object' - the 'thing' that is contracted cannot be criminal, tortious, or against the prevailing public policy. You will find

Contracts must have 'legality of object' - the 'thing' that is contracted cannot be criminal, tortious, or against the prevailing public policy. You will find that in each of your examples that there is a particular statute legalizing that activity in a particular state.