Morally? No. It's the thief's fault, and the thief's fault only.

But if you know that people steal packages off of porches in your neighborhood, and you leave a package out there for weeks, you're at least being pretty unwise. You're not morally at fault, but pragmatically, yes, you kind of are.

[Edit: That is, your actions are not well-suited for the kind of world that we actually live in, and that you know that we live in.]