People who avoid certain non-Kosher food only.

I am talking about avoiding pork and/or maybe shellfish without keeping seriously Kosher. There are some people who are not “frum” by any stretch, and realize that kashrut is more than this, but avoid some foods like these rather than saying, “well I’m not devout and not going to be strictly kosher, so might as well eat anything.” How common do you think this is? Is it maybe more common among older people? How should we understand it?