If the militants cross from Pakistan (if ever it is true).. then Pakistan's responsibility stops there.. once they are inside Iran it is Iranian responsibility.. Anyone can see on youtube videos how difficult it is to control drug trafficking near those borders..which represents a weakness to both sides.. So blaming Pakistan and extending the blame to others who do not even share borders with Iran look and sound very irrational..
They cross from Pakistan and are free to operate in Pakistan. So it is Pakistans responsibility. Its impossible to fight these terrorists when they can freely operate, train, recruit etc in a neighbouring country who provides a safe haven to them