There's nothing to clarify.

Creative work (including source code) automatically falls under "all rights reserved" bucket unless the author specifically states other licensing terms.

In this case the author didn't so his work is still "all rights reserved" and no-one can use it.

You're confusing "source code available" with "open-source".

"Open source" is a shortcut for: "source code available and under open-source license".

What license is an open-source license? OSI has a detailed checklist: http://opensource.org/docs/osd

(nitpicker corner: to the extent a vague term as "open source" can be defined; people disagree as to what exactly it means).

Under the Berne convention, all works produced in any signatory country are automatically subject to copyright: http://en.wikipedia.org/wiki/Berne_Convention_for_the_Protec...

Wikipedia says "A typical software license grants an end-user permission to use one or more copies of software in ways where such a use would otherwise potentially constitute copyright infringement of the software owner's exclusive rights under copyright law." http://en.wikipedia.org/wiki/Software_license

You may be basing your prior belief on e.g. Wikipedia's claim (scroll down) that "A primary consequence of the free software form of licensing is that acceptance of the license is essentially optional — the end-user may use, study, and privately modify the software without accepting the license." IANAL, but this statement seems dangerously incorrect to me. It may be technically correct that you do not "have" to accept the license terms--but then if you are downloading the software, or creating derivative works without a license, you would be violating the law. E.g.

Uploading or downloading works protected by copyright without the authority of the copyright owner is an infringement of the copyright owner's exclusive rights of reproduction and/or distribution. - http://www.copyright.gov/help/faq/faq-digital.html#p2p

It is true in the sense that there are conceivable ways to use FOSS without accepting the licence. There are common ways like dual-licensing (if the license is GPL and the company has all the copyrights they can still sell me a different license) or even fair use (someone could create a parody of FOSS and publish it).

> I would have guessed that without an explicit license, open source is free for anyone to take/modify/reuse.

That would only be true if the copyright holder explicitly places their creation in the public domain.

Open source does not mean 'I get to re-interpret copyright law in a way that suits me'.

How do you know it is open source without a license? Sure you can see the source code, but I wouldn't have said that is what makes it open - its the license that does.

The normal rule is actually to assume that all rights are reserved.