It is a relic of the old UK indentured servant system, where the only funds an indentured person could have to ever pay off his indenture was tips from visitors

It is a relic of the old UK indentured servant system, where the only funds an indentured person could have to ever pay off his indenture was tips from visitors. Later in the USA, this expanded to add slaves who gathered tips to gain freedom as well as indentured servants from the UK who often came to the USA on 5 year work indentures - unless of course they added to the debt at the 'Company Store' - to whom they owed their 'souls'