That's just false. The clerks are not required to "be agents of law enforcement". If the state required the clerks to arrest any minor who tried to buy cigarettes, then you would be right. The state requires employees to obey the law, and that's just fine. Also, the penalty goes against the store, not the clerk. These establishments need to make it abundantly clear to their clerks that they are not to sell tobacco to minors. If they do things that cause the business to be fined, they should be fired for it.
Why not just legalize tobacco?
Not the ultimate solution to the nanny-state neoprohibitionist movement, but it's a good start.