It's actually not illegal in the US. There's a very specific delineated list of reasons you can not discriminate in hiring (
Wikipedia), most of which are conditions outside of your control (age, genetics, etc). Anything else is fair game, if sometimes sleazy.
Hiring is an inherently discriminatory process. The whole purpose of an interview is to find the best candidate for a job - the most dedicated, the smartest, the best personality, and so forth. That's why at an interview you have to play yourself up: you have to appear to be the best. Appearance
does matter. Unfortunately, people with overly addictive personalities have cast a bad light on people who play MMORPGs and WoW in general.
What surprises me is that, at an interview, someone would answer that question. It's like being asked if you drink - why would you answer that at all, since it has (in theory) no bearing on your ability to perform your job. Answer it with a response that indicates you put work first (on the weekends, in my free time, when I have time, etc.) and move on. Or, simply state personal life and professional don't mix.