Because it's presumably beneficial. It gives your lawyers extra time to prepare for the case or to potentially settle on more favorable terms.
Plenty of cults have slaves in the US. But because the are willing, nothing is done about it.
"You know the worst thing about being a slave? They make you work, but they don't pay you or let you go."
"That's the only thing about being a slave!"
So yeah, you can legally be a slave as long as you leave out the one part that makes it slavery.