I think we would all agree that taking a complete project's worth of code from an employer and giving it to another company would be a gross violation of copyright. That isn't what we're talking about here.
People who I interview routinely bring me code snippets from previous employers. These are on the order of "this chunk of code searches a big text file for these fixed-width fields and sorts the records on this key" or similar. They are nearly meaningless without context, but are plenty good enough to get a sense of the person's basic coding practices.
I would never ask someone to bring code from a previous employer, but I do ask them to bring code samples, and that's usually what they bring. Some of them bring CPAN stuff, or code they write specifically for the interview that is tailored to the tools listed in the job ad (a good strategy if you can do it), but most bring stuff they wrote on the job, and I'm fine with it.