Because those folks are morally confused. They have no problem with killing off the "little piggies," to quote Tereza Kerry. But they realize that, being morally facile, they might find themselves behind the eight-ball in such a criminal conviction and sentencing. Thusly, the life they clamor for might someday be their own.
Obversely, there are those 99% opposed to abortion who also favor the death penalty. Their logic? Criminals deserving of the death penalty had their chance at life and blew it; the aborted unborn never had the chance...(nutshell analysis).
This approach can be expounded on for paragraphs, but you get the idea.
"Morally confused" is putting it kindly..My question was posed rhetorically, to point out the obvious..but you did a far better job than I..
So true!! They also are the first to support not eating animals, not hunting, and the lastest, cruelty to animals in farming.