I've introduced this, because Openfire was not XMPP compliant in this point and there are efforts to make it compliant (see OF-630).
See http://xmpp.org/rfcs/rfc6120.html#streams-ns-stream
I've never heard, that Flash clients are not compatible with the XMPP spec.
ejabbered obviously had the same problem: http://www.ejabberd.im/node/4083
I've fixed it by adding another check for the Flash namespace. Now, only if the namespace is neither "http://etherx.jabber.org/streams", nor "http://www.jabber.com/streams/flash", the stream is closed.
You may want to review the fix:
https://github.com/igniterealtime/Openfire/commit/da8b960bbe3e51e1019ae46d117a91 d5b6e40a53
I haven't tested it with a Flash client, but this fix should work. Maybe someone can test it with the next nightly build.