It is a typically conservative method of attack - attach a label (or negative image) and make it stick. They define their enemy using stereotypes if they can. A lot of people have negative stereotypes of D&D players. It is pretty much the same sort of demonization many super-religious people do to people they disagree with.
Here is an interesting article on why Obama needs to fight back harder against conservative smears, including a look at historical attacks of this nature:
http://www.newsweek.com/id/153323
In another article (
http://www.newsweek.com/id/154058), the author shows a "smear gap" between Obama and McCain, positing that McCain is much more negative in his campaigning - and doing a lot more straight-lying than Obama in order to define Obama and make it stick.