Once a project is shared on the Scratch website, the creator of the project agrees to license it under a Creative Commons Share Alike license. This means that everyone is free to download, remix, and reuse it as per the terms of this license. If you do not want others to view and remix your creations, you may not want to share them on the Scratch website.
We encourage remixing in the Scratch community, since we have seen how remixing can lead to creative, interesting projects. The website includes a 'Thanks to author for the original project...' link to give credit to the original author -- and we hope that remixers will also give credit in their project notes. Overall, we want the Scratch community to be full of creative activity and exciting projects, and remixing helps with that.
We appreciate you helping to keep Scratch a friendly and creative environment!