Let me start out by saying that I think opening an issue isn't all that spammy to me. It's certainly better than emailing directly, although it still generates an email, so it's still "kinda spammy" IMHO.

From a philosophical standpoint, I'd say it's no one's place to be automated cold calling (cold issuing?) other developers to explicitly license their software. If there is no explicit license, the implicit copyright still exists. GitHub is really the only party that would have an understandable role in these kinds of warnings/requests.

Here's a good SX post on the topic: http://programmers.stackexchange.com/a/148165