I actually found a reason for this at some point, but didn't save it, and now can't find it. Something about the acrylic in fake nails can break down into potentially toxic by-products when combined with the alcohol-based hand sanitizer we use like mad in the hospital. At my hospital, everyone (including nurses) who handle drug products are not allowed to wear artificial nails.
Nail polish, however, has never been a problem for me, in retail or in the hospital. I wear it religiously to keep me from biting my nails. We are told to stick to "conservative" or "natural" colors but no one's ever complained if I go festive for a holiday. One pharmacist did point out my nails when I tried black polish once but that was more because we were waiting for a state inspection rather than she really cared what color my nails were. I agree with others, keep it well-groomed, not chipped, no matter what color it is (light colors you can get away with this more; dark colors look bad as soon as they start to chip).