Most everyone here was once a guest first -- boards that allow guests receive better traffic. We are not old enough, nor snooty enough, to require everyone to register first.
Keep in mind that the guests that flame are usually members that log-out first. They are rarely new guests. Mods do warn these incognito members and delete the offending posts. However, don't assume we have time to read every single message on the board. The best way to handle this is to simply pm a mod with the offending text and a link to the thread so we can deal with it.
Call it from Heaven Chick.