"They were punishing the robber."
Most dictionaries include the use of force as integral to "rob", but it doesn't have to be personal or violent. One can rob a museum by breaking and entering when no one is there. English uses the words very flexibly and with a lot of overlap, but I have decided that the actual distinction is that the object of "rob" is a person or place and the object of "steal" is the valuable object that changes possession.
Since people and places usually require force for you to take something, that sort of matches the common definition. But even if no force was used, if we find something missing we say, "Somebody robbed me. They stole my wallet." So the difference in object is the distinction I assume the Klingon to have.
When adding -wI' we don't always have an object, so it can be difficult to determine which is the better word to use. In reality, all stealing requires an owner, a location, and a stolen valuable, so when used without an object, I'm not sure there really is much of a distinction.