Pretty soon, we'll hear about companies who won't interview employed workers, because obviously if you're looking around while you've already got a job, it means you're not a loyal employee.
Furthermore, it does happen somewhat already with the "job hopper" label. You've only been at bigco for 18 months and you're already interviewing someplace else? You'll have no loyalty to our company!
Yeah, that always gets to me, for some reason. "You expect me to demonstrate loyalty to you when your corporation is not willing to demonstrate the same loyalty to me? Yeah, right."
Of course, what I actually say is, "I really enjoyed my time at BigCo, and I learned a lot there, but I'm looking to broaden and deepen my skills. Your job, because of <differences x, y, and z> seems like it'd be a good fit for that."