Das ist der vorübergehende Standard von den Upstream-Devs.
Zitat:
With Postfix 2.8 and earlier the default is "ipv4". For backwards compatibility with these releases, the Postfix 2.9 and later upgrade procedure appends an explicit "inet_protocols = ipv4" setting to main.cf when no explicit setting is present. This compatibility workaround will be phased out as IPv6 deployment becomes more common.
Hier nachzulesen:
http://www.postfix.org/postconf.5.html