I do not understand your analogy. Nokia is responsible for official gas tank with official tank cap. If you choose to change it or make modifications to it then you are on your own. This is how real life works!

You can brake N900 by single command from x term. You are able to brake it in many different ways. Just install stuff from webpage x and BOOM there goes dynamite! You are responsible 100% on what you install on your system. I would say that If you only install stuff from Nokia repositories and it brakes then you are entitled to contact nokia care. But if you choose to fool around with your device because it´s possible then you are on your own. Simple as that. I do not know if this whole responsibility thing is too hard to understand. If so then I´m really really worried about my fellow internet warriors around me :|

Should hammer have some kind of security method that prevents user from hitting himself/herself? It´s probably possible to make security method that makes it safe but it comes with cost of easiness of usage.

.edit
OH and BTW When people are using Nokias own application manager to install stuff from maemo.org please READ the warning text. It´s not there just for fun.