Really good question, and it's one of several potential problems with Libertarianism.
So, at face value, you're right - you do. But then, you can also form organisations within your community to do the same. So then your whole village or street enforces the NAP.
Besides which, it's not really about other people, it's about you.
I won't hurt you or nick your stuff.
Yeah that was shithaus. They should have just apologised and explained that it would be on the new gen hardware, and could have avoided all that.