The Daesh have declared war on their own people and the U.S. As long as a hospital and school were nowhere nearby, then a radio station that promotes the propaganda of the enemy is fair game. We can't put a choke-hold on ourselves whenever we are confronted with the enemy.
U.S. and Afghan forces were justified in ordering an air strike on the Isis-run Voice of the Caliphate radio station because the spread of terrorist ideology must be stopped. All personnel operating the radio station were militant. U.S. and Afghan forced should continue to destroy terrorist communication hubs such as this one.
One of the fundamental ways that humans can work with each other, make plans, and carry out those plans, is through communication. A radio station is a way for ISIS to communicate to its members and also recruit new members to help coordinate attacks on innocent lives. The means justify the ends.
If the proper due diligence were done in choosing the correct target and no civilians were to be hurt in the strike then they were justified in ordering an air strike. If ISIS is going to continuously use violence and murder to conduct their business then they should expect the same in return.