I'm not sure why it is that some websites think they can make up their own copyright law.
In the US copyright and other intellectual property law is made by Congress. Our Copyright Act makes the rules, and doesn't say that individuals or businesses may make up the rules for themselves. There is also the international Berne Convention on copyright, of which the US and Japan are both signators. The following describes US law (since that's what I'm familiar with), but I'd expect Japanese law to largely be similar because of the Berne Convention. Words in parentheses are taken from the US Copyright Act, which defines them.
An "author" has a copyright as soon as his work is "fixed in any tangible medium of expression." A child has a copyright in his crayon scrawls, and a shopper has a copyright in her shopping list.
An author may assign all of his copyright to another party, or grant an exclusive "license," but ONLY by signing a written agreement that says so. But lesser rights (a "non-exclusive license") may be assigned orally or even by implication. By posting this here I'm granting the site-owner a non-exclusive license to display this post on this site.
One problem I see in the opening post is a confusion between ideas and the expression of those ideas. Only the latter is subject to copyright. In Western law at least, ideas are considered "free as the wind," to quote one old court opinion. Unless you disclose the idea under some type of non-disclosure/non-use agreement, that is.
I would never post on a webiste any idea I was considering writing into a work. And I don't post any work I consider marketable, just because some pirate might take it, or because some person acting lawfully (however unscrupulously) might steal the idea expressed by that work. Also, posting something on an internet site would be considered a first publication by many print publishers, and would likely hurt the value of the work. I know magazine editors who won't consider buying any article or story that's already appeared anywhere on the internet.
Roland