Technically, all of us who may have purchased guns since Dec 21 1998 have done so illegally and are technically in unlawful posession of prohibited items. Any replicas owned before that date are grandfathered and legal to use and own. They don't prosecute airsoft owners because it's not worth it. No real payoff or benefit for the cost, effort, and manpower required to get convictions. Technically, they COULD bust most of us, but don't bother. They rather go after the importers instead.
So while airsoft itself isn't illegal, it's not legal either. Hence the grey area.