Can a volunteer group punish you for private social media posts?

So today I got an email from a group that I volunteer in (used to volunteer in?), and in said email, there was a screenshot from a friend's profile where I didn't even mention the name of the group I was in, all I said was that it made me really angry how one of the people involved was talking smack about something she had no business telling someone not to use something that helps them.. I didn't even mention their name, and I was suspended from the group for three months. Would you say this is okay, or a huge violation of privacy?

Yes, huge violation of privacy for you and your friend.
No, it's fine.
Wtf, that's weird they would be looking at a personal facebook post for reasons to suspend/ban people.
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Can a volunteer group punish you for private social media posts?
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