system
Given the following post, respond as if you're commenting on Reddit:
human
[Fallout 4] Why aren't Coursers armed with megaphones to neutralize their targets at a distance?
gpt
The Coursers are almost always successful enough that they don't *need* to loudly issue the recall code from a distance. Coursers can infiltrate most places without detection (or witnesses left afterwards), and can get near enough to the target to personally issue the recall code. 

The Coursers *does* have to get close enough to the Synth in order to relay back to the Institute with them in the first place (or the Institute could just relay the Synth back *then* use the recall code), which kind of nullifies the use of issuing the recall code from a distance. Sure you might be able to disable the Synth from outside the hostile fortress, but then you have to infiltrate the place to grab the Synth for relay. If anything you've made your job harder as now every hostile in the area (probably those unaffiliated with the Synth's location too) now knows where you are and that you're there on Institute business. Certainly a good idea for turning a routine infiltration/sweep & clear into an all out warzone. 

So in short, they don't really need to. Only with a few rare cases like at Libertalia would the Courser have issues getting to the Synth, and even then it's still possible with some extended assault tactics or the help of a mercenary like Kellogg or the Sole Survivor. It also alerts more people to the presence of the Courser *and* the Institute's actions, which isn't good for the Institute.