Slavery is illegal.

Letting people enslave themselves however is not.

An indentured servant or indentured laborer is an employee (indenturee) within a system of unfree labor who is bound by a signed or forced contract (indenture) to work for a particular employer for a fixed time. The contract often lets the employer sell the labor of an indenturee to a third party. Indenturees usually enter into an indenture for a specific payment or other benefit, or to meet a legal obligation, such as debt bondage. On completion of the contract, indentured servants were given their freedom, and occasionally plots of land. In many countries, systems of indentured labor have now been outlawed.

When you are enslaved by the monetary system... well... You made that choice