While I agree that Delta should do a better job of alerting their passengers, they probably have a lot of other stuff to worry about. It doesn't seem entirely unreasonable to expect people flying in and out of NYC to know they shouldn't bring guns. Now when people get stuck in NYC it's a different matter.
Child pornography is another matter. It is self-evidently criminal. Not so with guns or substances.
Most of the time I'd agree with this 100%, but in NYC a gun in the posession of someone from SD is self-evidently illegal.