From my own little language of code and GitHub, I believe all open source code is free to use by anyone. Isn't that correct?
This isn't true, without any license it only means the source code is available. Even with license, usually there are few limitation and obligation. Here's snippet from GitHub documentation,
You're under no obligation to choose a license. However, without a license, the default copyright laws apply, meaning that you retain all rights to your source code and no one may reproduce, distribute, or create derivative works from your work. If you're creating an open source project, we strongly encourage you to include an open source license.