Better to completely shut the door and bolt it closed than to open it and lose more freedoms.
That hypothetical argument you put forward will never come to pass because:
1. Contrary to popular belief nuclear bombs can't just go off willy-nilly. They require perfect timing, where a whole range of factors have to line up perfectly before the atoms will even split. If it's not detonated within that perfect time-range, it just buries itself in the dirt and starts producing radiation poisoning.
2. You can't execute someone as a criminal if they've not even committed the crime yet. That's just barbaric.
3. Once due process is abandoned for one type of crime, it's just a matter of time before it's abandoned for all types of crime. Why give the president a free pass to start that ball rolling?
4. Drone strikes are a military measure. Unless the US is at war with foreign powers (with an actual declaration of war being passed through both houses of parliament), they have no reasonable justification for using them at all, let alone on their own citizens.
5. "Last resort" options, once resorted to, tend to become the norm over time. Is that really the norm that you want?
6. Who determines whether or not the intended strike victim is actually a threat? I realise the point you're trying to make is that stopping the guy in your example would save lives etc, but as soon as you justify its use against Americans on American soil once, then it's up to the current administration to determine whether or not future drone strike victims are actually a threat. The way that the US is going, it's just a matter of time before someone usurps the role of president and becomes a Soviet-style dictator (you're almost there now, actually), and then where will that leave you guys with the drones? Answer: Political opponents and those who are very vocal in their condemnation of such things will be executed using those drones by those in power to silence them. Better to outlaw it period now than to lose even your own security later.