Nobody is being held at Arik to perform work. While the pilots aren't being paid, each day they wake up with a passport in their pocket, they are free to go home (if Nigeria is not their home).
Pilots enable this company's poor behaviour by continuing work without wages. A far cry from slavery. Quite the opposite, actually.
Why would they pay, when you'll continue to work? Sounds like good business, Nigerian style, to me.