Part of the definition of “freedom” in the open source world is the ability to do whatever you need/want to do with the thing in question. Some of this is about immediate needs, some of it is about the future. Consider, for example, what would happen if pianobook.co.uk folded one day, but you wanted to collaborate with someone using the (marvellous) Winter Voices library. You have a copy yourself, but you’re in violation of the terms of agreement if you give it to them. Similarly, if there is another “tectonic shift” in sample library formats in the future, and someone wants to repackage the samples in a PB library for some new format, that’s problematic right now.
There’s been a bit of a tradition in the audio tech world to generally ignore this kind of thing – hence the bazillion cracked copies of your-favorite-DAW or your-favorite-plugin that just roam around. The open source world tends to care quite a lot about trying to both (1) honor current license arrangements (2) use license arrrangements that are “future proof”, both in terms of legality, but also functionality. It’s one thing, for example, for the company that produced Bias Peak to stop making it anymore. It’s something else to no longer be able to even use Bias Peak because nobody can keep it working on later operating systems. Sample libraries might seem initially as if they are immune to this sort of problem, but if you think about harder, I think they’re more similar than one might suspect.