Right now NO ONE. Crazy right? Well not really. If you forced people (satellite owners) to pay for someone to deorbit their stuff, before an affordable service exists, that'd be a quick way to put a major damper on the booming space economy. It is crucial to develop the technology quickly, and do so with a system that is affordable. Only after this will regulators be willing to move towards enforcing more strict policies for deorbiting your satellite if it dies. Until then, anyone need an orbit modification? :)