My collected rationales for placing these libraries
in the public domain:

1. Public domain vs. viral licenses

Why is this library public domain?
Because more people will use it. Because it's not viral, people are
not obligated to give back, so you could argue that it hurts the
development of it, and then because it doesn't develop as well it's
not as good, and then because it's not as good, in the long run
maybe fewer people will use it. I have total respect for that
opinion, but I just don't believe it myself for most software.

2. Public domain vs. attribution-required licenses

The primary difference between public domain and, say, a Creative Commons
commercial / non-share-alike / attribution license is solely the
requirement for attribution. (Similarly the BSD license and such.)
While I would *appreciate* acknowledgement and attribution, I believe
that it is foolish to place a legal encumberment (i.e. a license) on
the software *solely* to get attribution.

In other words, I'm arguing that PD is superior to the BSD license and
the Creative Commons 'Attribution' license. If the license offers
anything besides attribution -- as does, e.g., CC NonCommercial-ShareAlike,
or the GPL -- that's a separate discussion.

3. Other aspects of BSD-style licenses besides attribution

Permissive licenses like zlib and BSD license are perfectly reasonable
in their requirements, but they are very wordy and
have only two benefits over public domain: legally-mandated
attribution and liability-control. I do not believe these
are worth the excessive verbosity and user-unfriendliness
these licenses induce, especially in the single-file
case where those licenses tend to be at the top of
the file, the first thing you see.

To the specific points, I have had no trouble receiving
attribution for my libraries; liability in the face of
no explicit disclaimer of liability is an open question,
but one I have a lot of difficulty imagining there being
any actual doubt about in court. Sometimes I explicitly
note in my libraries that I make no guarantees about them
being fit for purpose, but it's pretty absurd to do this;
as a whole, it comes across as "here is a library to decode
vorbis audio files, but it may not actually work and if
you have problems it's not my fault, but also please
report bugs so I can fix them".