Because until recently, the library was license-free.
Licenses are important as a concept. Licenses can exist, and when they do, they must be honoured. They are an aspect of the property rights of the owner of the software.
That doesn't mean you have to use one, and the library was not using one, so it was license-free.
There might be some confusion here in that when I speak of licensing being important, I mean the concept - I do not particularly mean licensing as it exists today in practise.
As was pointed out in an early post in this discussion, the matter is now not so clear cut. I've added per data-structure information on licensing, and it's no so straightfoward a situation now as to be able to say, in a blanket fashion, "license-free". I'm thinking about what to do about that. The library is much closer to license-free than anything else, but it's hard to explain the situation in the one-line description while keeping it short and to the point.
Why don't you just license it under a single free software license and call it a day? Why say "you have an infinite set of dual licenses"? If you really, really really don't care just use CC0.
I don't understand your focus on the "concept of licensing". It's such an odd thing to focus on with such vigour. In my mind, a license is a tool that allows me to ensure that users of my software have freedoms that they deserve. Some people use it as a tool for oppressing users. But licenses are a tool, simple as that. They are a consequence of copyright law. Copyright licenses have literally nothing to do with property rights (you can't put a copyright license on a goat or a piece of land).
Licenses are important as a concept. Licenses can exist, and when they do, they must be honoured. They are an aspect of the property rights of the owner of the software.
That doesn't mean you have to use one, and the library was not using one, so it was license-free.
There might be some confusion here in that when I speak of licensing being important, I mean the concept - I do not particularly mean licensing as it exists today in practise.
As was pointed out in an early post in this discussion, the matter is now not so clear cut. I've added per data-structure information on licensing, and it's no so straightfoward a situation now as to be able to say, in a blanket fashion, "license-free". I'm thinking about what to do about that. The library is much closer to license-free than anything else, but it's hard to explain the situation in the one-line description while keeping it short and to the point.