Automatically join a group when post a post on group's public forum using buddypress 1.2.5

Is this behavior normal? When a user sign up with buddypress 1.2.5, or a user that is already sign in, he/she has not yet joined the group, goes to a group forum, adding a post on the group forum, he/she suddenly is becoming a group member of that forum's group. Is it normal? I thought a user has to join the group first before he/she can post anything on a group's forum, and to think of a user just have to post a post on a group's forum to be automatically joined the group is a little different from what buddypress behaves. So is this a new feature of buddypress 1.2.5, or it's a bug?

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