Globally Unsubscribed?

I have had several clients mysteriously become "Unsubscribed" from their own lists. Now I know why. I myself recently unsubscribed from an unrelated emailing list that apparently was being sent out through Constant Contact. But instead of simply unsubscribing myself from THAT list, it apparently unsubscribed me from ALL Constant Contact mailings. I had been removed from my own outgoing lists. Resubscribing myself was easy. That's not the issue. But it seems inconceivable that just because someone wants to unsubscribe from a mailing list for, say, a restaurant in Toledo, that such action results in them being unknowingly unsubscribed from every Constant Contact list they're on, even the ones they very much want to continue receiving. Big flaw that needs to be corrected.