[scurvy]

I'm really confused here. Why would a DDoS mitigation service announce routes for a network they don't serve for a customer? That would simply be an old-fashioned BGP hijacking.

Transit providers don't simply send traffic to Prolexic and Defense.net because they think they should. They send traffic there for routes that the mitigators are announcing. They'll only announce client routes (and only when clients announce to the mitigators).