Because all the packet plugins are loaded by the core, not by the Battle.net plugin, so they'd be getting the raw socket data instead of packets. The only issue with that is that Battle.net is known to send packets in pieces or several packets in one clump, so there'd have to be a non-protocol-agnostic packet thread to pick these apart for parsing, not to mention it's not a good idea to trust TCP connections to have packets split perfectly, even if it's known to in the past.