I think 'slavery' is too emotive a word.
They were imported as "unpaid labourers" that's all.
Anyway the massas gave 'em free passage on ships from the heathen jungle to Christian America, fed 'em, clothed 'em, housed 'em, gave 'em steady work and taught them to speak English, all of which was payment in itself.
They weren't paid in money because there was nothin they needed to spend it on anyway.
To sum up, the slavers did them a favour by rescuing them from the jungle where they'd probably have ended up in the next-door tribes cookpot, and bringing them to America to civilise them..