Being "nice" is easily faked. You can hate someone and still smile and be nice until they leave your store/presence/area. This sort of behavior just makes life easier in a lot of ways. For instance if you are working at a store and a really obnoxious person asks where something is it's probably best to show them rather than argue with their attitude because that will get them out of the way quicker.
Being kind on the other hand requires the kind person to actually care about whoever they're being kind to. It'd be like someone giving money or food to a homeless person on the street. Someone faking being "nice" isn't going to see any reason why they should do something for a homeless person they don't know and may never see again. Only a kind person would do this - out of empathy for another human being.