I stumbled upon an interesting paper (PDF Link) where researchers found out that more than 58% of the DNS open resolvers that are blamed in DDoS attacks are in fact home routers distributed by ISPs to customer. The paper goes on to find out the reason for the odd (anomalous) behavior where these routers respond to DNS queries with the wrong source port (not udp 53), and found that the problem is probably caused by a faulty NAT rule.
I think that research is interesting, but what I cannot understand is that why would manufacturers make such home routers accept any kind of traffic initiated from the wild internet? I know some of them have the option to open configuration page from WAN thus listening on port 80 or 443 (or whatever custom port you set), but what justification can be thought of for opening up a service like DNS to the outside world? And if that was just a side effect for something those manufacturers tried to accomplish, what would it possibly be? Could it be some kind of a backdoor not configured well? and to do what? Or am I missing something here?!