Baku opted for a warrior instead of magician? That's intriguing. I wonder just how many of the slaves stayed for the sake of the gamble. Is it just those 6 that fought that random level 80 guy?
Pure speculation at this point, but it doesn't seem too "difficult" to become a slave, seeing as Champ became one after losing his bios, and the same thing happening to the Azura slave's because of Lalo. Maybe there's like a tax or upkeep system that forces them to become a slave if they don't meet it? So Baku can just pretend he's broke, and have someone else hold his bios (in case they check his vault).