> If you don't know if you can distribute it, you should assume you can't
That's 100% wrong. The default state of a work of art is to be in the public domain. Copyright law creates only a limited exception to that general rule.
That's incorrect in any country (167 of them, including most of the west) that signed the Berne Convention (http://en.wikipedia.org/wiki/Berne_convention), which makes all works copyright-by-default.
That's actually incorrect in the US. The default state of a work of art is that it is copyrighted with ownership granted to the creator, with all rights reserved.
These rights have to be waived for it to become public domain.
The Creative Commons level zero license is probably the best option. In some jurisdictions it is impossible to give up something into the public domain, so the CC0 explicitly gives up as many rights as possible. If it is possible to put something into the public domain, then it does.
Morally speaking you might be right, and on geological timescales, sure. But every work is under copyright the moment it is created until copyright expires unless the creator specifically gives it to the PD.
In the modern world, unfortunately, if there's no accompanying license with the material it's pretty likely that you don't have the legal right to distribute it.
Not really. Works are automatically copyrighted upon creation. It's not some limited exception. You cannot assume a work is public domain until proven otherwise. Otherwise you'll be getting a nastygram from the RIAA.
That's 100% wrong. The default state of a work of art is to be in the public domain. Copyright law creates only a limited exception to that general rule.