If you allow a browser to open sockets to random places, it's the exact same principle! The onus is on the listener to ensure that the people communicating respect the agreed upon protocol.
Then every browser could be used to send spam. It would be horrible. Imagine that every person who visits a compromised page immediately started sending spam emails over smtp. Or spam irc messages.
Generally is not a problem, as Flash needs to check a file called "crossdomain.xml" that is served from the destination server, which specifies how Flash can communicate with it.
