Where is the line drawn for ethical hackers using stolen credentials in their paid services?

The very interesting question I have is when “ethical” hackers/pen testers harvestthese repositories of stolen credentials to then use them in pen testing for paying clients what ethical boundaries are broken? What laws are broken? If a lazy hacker leaves their captured credentials out on un insecure, public facing server and then an “ethical” hacker grabs them for their own paid services, it seems to me that it’s stealing already stolen goods.

What about a penetration tester taking credentials gathered from a paid/contracted job and adding them to a database to be used in future client jobs?