I would reiterate what Iranzamin has said.
1. These people were on the Iranian soil. so Iranian laws can be applied to them and because they were ordinary citizens and not Pakistani officials I doubt Pakistan could condemn Iran for that based on legal grounds. the same is true if the same case happens to Iranian civilians crossing into the Pakistani border and I say that from the perspective of law.
2. Some people think that Iranian guards should first arrest them and put them on trial. That's not how we do it. It's our little rule for the world. If you cross the Iranian border and you don't follow the instructions of the Iranian border guards, it means you have ill-intentions. When you're dealing with drug smugglers you don't go and talk to them like "Hello, May I check your identifications please?" and then if they were smugglers you would go on and say "sorry, but according to our laws you should come with us to the police station". That sounds more like a joke. Iranian border guards warn them first, if they don't head the warnings, they'll be shot down. It's just as simple as that.
3. We have lost more than 20,000 innocent soldiers in the last decades fighting with drug smugglers. Most of those soldiers were ordinary people who were serving in the Iranian military for an obligatory 20-month period. (It's actually a much shorter time for these soldiers because they are serving in a 'dangerous' zone). We don't want to risk our soldiers' lives. No country wants to risk the lives of its forces. Drug smugglers should just forget about coming to Iran if they like to stay alive or else they'll be confronted because they are not welcomed.
What bothers me the most is that some people want to make these drug smugglers look innocent civilians who have unintentionally crossed the border and evil Iranian border guards have shot them down. An innocent civilian will head the instructions of border guards if they like to stay alive. An innocent civilian, even it's highly unlikely that he/she crosses the border unintentionally, will know that border guards can shoot them. I, as a civilian, know that if I cross into another country's territory they can bust me or even shoot me down. If some people still don't know that simple fact, I think they should be taught about it on the media.