Because it's clearly a war crime.
They already released the video of the first strike. The second strike came 41 minutes after the first with the survivors trying to signal for rescue since they'd be facing near-certain death if left alone on the high seas. That's when "Operation Kill'em All" kicked in.
FWIW, a Hellfire missile is about $120K each. They used four on this boat alone. Why not save the money and just pick them up along with the drugs?
Maybe it's confirmation of that saying that "when you only have a hammer, everything looks like a nail"? 41 minutes? Is this confirmed?

