Indentured servitude was a form of slavery entered into by an initially voluntary act, blue faces not withstanding.
If they voluntarily agreed to be an indentured servant for a certain amount of money, for a certain amount of time, but then were not let go, that would be fraud, because the indenturer didn't live up to his end of the contract.
It's hard for me to imagine how awful people's lives must have been back then that they would do such a thing as sell themselves into servitude, but they must have thought it a better deal than whatever would have happened to them (or perhaps, their families) otherwise. They're the ultimate example of people trading liberty for security.