If someone publishes their sketch on this forum, does this constitute a public release to everyone who reads the post? That doesn't sound right, but neither does the alternative.
Authors of code automatically have copyright over their creation. That means, if they expect it to be shareable, it should at some point get an attached notice about the exact terms.
In particular, if someone posts code with a "can someone help me get this working" comment, that doesn't mean that that code is automatically shareable. A reply containing some clever idea should be shareable (for the piece of code provided.)
Code should have a license attached if you put it in the "playground."