I think people are mixing two things together here. If we're talking about full time employment, it's safe to assume that whatever work you did is not shareable, absent an agreement to the contrary.
For consulting/contract work, things are more complicated. When I was doing consulting, I tried to avoid signing agreements giving my clients ownership of the code. If they insisted on it, I made it clear that much of the code would probably be generic and generally the same as previous projects I'd done, and that I'd reuse similar pieces in the future. I also made them sign an explicit disclaimer on all my free software projects.
So if you're doing consulting, then I'd say that being asked to produce code samples based on previous work is entirely reasonable. But the original post sounded to me more like a full time job situation.