I think the reason it isn't a rule on any forums I've come across is because it's unlikely to happen. It's a rather farfetched situation that a store owner would come across a negative review, and attempt to sue the forum it was posted on.
I completely agree, the chances of it happening are very small, but it covers the forum admin/owners in case that chance ever happened.
And, I don't think you need to be sorry about going OT. It relates back to why the name was removed from the original post, and so makes sense.