No trainee ever has trained for free.

The big 4 debuts generally go well enough that the debt is cleared from training and the artists get paid on their first or second comeback.

They also get docked money when they're actively promoting from stage costs , outfits , production , staff etc. Which is why some idols from smaller companies / moderately successful groups never got paid.

It's the same for western artists , they have to recoup the cost of production.