Why is that unreasonable? If I write a complex piece of software, and something goes wrong, the blame is on me. As you say, there's no way to claim that it is without bugs.

Why, when I use someone else's software without so much as telling them, let alone signing a support contract with them, does the blame shift to them? My responsibility is to deliver a service, not to write software. If writing software is the easiest way to do it, great. If using someone else's software is the easiest way to do it, great. But I'm the one running the service, either way.