SocksPort IPv6 flags differs in default config and in Torlauncher prefs, and exits can distinguish them

By default tor daemon set only IPv6Traffic flag in client sockets, but not PreferIPv6 https://gitweb.torproject.org/tor.git/tree/src/app/config/config.c#n6094

But Torlauncher sets them both when launch tor daemon with Tor Browser https://gitweb.torproject.org/tor-launcher.git/tree/src/defaults/preferences/torlauncher-prefs.js#n41

As i can see in spec, this flags sets bits in "FLAGS" value in RELAY_BEGIN cell, and exit relays can recognize which flags sets in client port settings. So, exits can distinguish circuits from Tor Browser Bundle clients and, as example, from daemons in linux distro repositories, with high level of probability. https://gitweb.torproject.org/torspec.git/tree/tor-spec.txt#n1595

Is there any reasons why PreferIPv6 flag is not turned on by default? This issue raises up in past (legacy/trac#21269 (moved)), but solution is not efficient - anyway flags sent to exit by default differs from vast majority of real usecases - surfing web with TBB.