That's not his point.
The government has killed American citizens on American soil in the past and will likely do so again.
His point is that a drone strike is a logistically ridiculous, in fact a nonsensical, way to go about doing it.
But the true point isn't whether or not it's logistically feasible to kill an American citizen with an armed drone while sitting at a Starbucks...the problem is the legal opinion issued by Justice that says the President has the power ot order the murder of any American at any time without benefit of trial. Obama's claiming this power and the justice department is backing him up.