They have late fees and can have incredibly high interest if you pay late.

1 in 10 for $30 includes every single man, woman, and child in the United States. In reality there is a much smaller group spending much more money and developing a revolving door of debt where they are chronically in debt for non essential purchases and as soon as one of those payments goes late they will be slapped with a bunch of late fees and interest that will make them miss other payments and get even more late fees and interest until they end up underwater

Klarna got 2.8B in revenue last year and they did it with those “interest free” loans