Won't they hand them back to the pipeline owner who paid the ransom?

I don’t know, I can thing of reasons not to. They want to discourage companies both from lax security standards and from paying ransoms when they get attacked. Not paying them back would provide economic incentive that even if the DOJ is working to combat hacking, they won’t be saving you from your own incompetence.

Not saying that’s what they will do, just that I think it would make sense to me.