This is a horrible situation. Don't allow them to take most of your hard earned money. The money belongs to you as it would in any other kind of employment. They get away with this do to lack of enforcement, and because of intimidation, fear and lack of knowledge. Anyone who does this is a swindler.
Artists bring too much happiness to many people.
America is the land of the free, and artists are not supposed to be indentured servants. It is a form of what was known as white slavery and is illegal. It may also be against federal law if the acts are imported or exported across state lines for these fraudulent purposes. These laws were made to protect you from crooked agents. Remember, this is not a choice, it's the state law.