[Drolefille]

1) Because some topics are group specific, that doesn't mean that other members don't have things to add to the conversation. Many times we have more similarities than differences.
2) Because some topics are non-group specific.
3) Because some people choose poorly when making their thread.

It's the mod's discretion to maintain his or her forum. Once a thread is started, do not be surprised if non-members post in it. Particularly if it's a hot topic. Badge selling would be one of those. I suspect if someone posted about ritual collecting in a GLO specific forum there would also be much outside commenting. Some people feel more comfortable posting in others' forums, particularly since some forums are very welcoming, some people don't. That's your own personal preference and shouldn't be imposed on others.