US law is pretty similar in this regard, isn't it? If you don't have a license for a particular piece of code, you can't use it without the author's/copyright holder's permission, even if you found it posted online.

You're expected, wherever you are, to look into where any code you use comes from and what legal rights you have to use it. (The author not offering you a license means you can't use the code, nearly anywhere in the world - pretty basic Berne Convention stuff.)

This is the legal expectation in general, not just for software - you can't just come across a design for a neat widget somewhere and start using it in your product, there's probably both copyright and patent on it. Software isn't special. Not everything in Github can be copied into your code verbatim.

That’s how us law works too. Works are automatically under copyright, even if you don’t say so. It needs a license to lessen the copyright restrictions.