And why not just garnish wages???

I think Repo Men wages are dependant on how many repossessed organs they are able to collect.

This is evidenced by Jake's advice to him to "get as many as you can" when his contract was almost up and he took him to that place filled of runaways.

So, basically not working, Jude Law didn't have much of a salary to begin with.

The companies business model is they make the most profit on an artiforg if they can sell it more than once. The next most profitable is the payment plan, the least profitable is the paid-in-full-upfront option. So defaults and repo make they most money. They WANT people to default so they can get the organ back and resell it again to someone else.