"You haven't explicitly licensed your code under any license and so, to be conservative, third parties should operate under the assumption that the code is proprietary"
I would have guessed that without an explicit license, open source is free for anyone to take/modify/reuse. After all, it is open source.
In other words, the default would be the most permissive license. You suggest it is the opposite - that without a license, the code should be considered proprietary. Can anyone more familiar with open source licenses 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".
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).
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.
I would have guessed that without an explicit license, open source is free for anyone to take/modify/reuse. After all, it is open source.
In other words, the default would be the most permissive license. You suggest it is the opposite - that without a license, the code should be considered proprietary. Can anyone more familiar with open source licenses clarify?