These are items under prohibited/controlled item list under the
Police's 'Arms and Explosives' licensing laws.
Certain types of knives (eg. flip knives) have no other purposes
except to attack/kill people. So they are strictly prohibited and
no types of declaration/licensing will be allowed.
A warning means - they give him a chance + benefit of doubt that
he probably doesn't know its prohibited.
IF he attempts to smuggle it in again, he will be charged in
court. Simple.
They can take it that with a warning - it means there're no
further action from them anymore, unless he tries it again.