I wouldn't consider that to be not owning the engine. It seems to me that the airline does own the plane and the engines on it, and this is more of a usage-based support/maintenance/repair subscription service provided by the engine manufacturer. If you suddenly didn't want it for some reason, I don't think RR could or would force you to take off the engines and ship it back to them. They just wouldn't provide you with the support and service that you stopped paying for.