Because the constitution doesn't allow one, but does allow the other.
Uhm, no, the constitution specificaly prohibits depriving one of one's life, without due process.
No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury, except in cases arising in the land or naval forces, or in the Militia, when in actual service in time of War or public danger;
I guess that part must be missing from your copy of the constitution.
I guess the fact that we are not at war with Yemen, either declared or undeclared is missing from you awareness.