[danielvf]

In both cases there was no rulership going on, and in both cases the robbery being done was in areas not under the robbers baron's de-jure authority.

[thaumasiotes]

When you build your own castle and set up toll stations on roads and rivers, there is rulership going on. You're it!

The wikipedia article suggests that the robber barons weren't difficult to stamp out, because they lived openly in giant castles. They existed because there wasn't a government above them to squash them. Filling a power vacuum doesn't make you a bandit, it makes you a warlord.