They're often not technically pro-bono. Clarke, for example, gets paid by the government, because they have a vested interest in not having cases overturned on appeal due to insufficient counsel.

https://www.abajournal.com/magazine/article/judy_clarke_has_...

> Clarke would probably not want anyone to feel indebted to her. In fact, after the Smith case, she returned the $82,944 fee the state paid her, saying that other indigent defendants could use it more.