EIGRP and poison reverse

Today, Stretch posted an interested EIGRP update scenario.  I found myself confused, though, at Step 4 where it says "R3 has a learned of a new path to 0.0.0.0/0 from the reply sent by R2. First, it sends a poison reverse update to R2 for this route."  Why would a router send a poison-reverse update for a route to a neighbor whom originated the route?  Cisco sheds more light

Thanks Stretch.