I couldn't even imagine...

That's because you're approaching this thread as an argument rather than an educational opportunity.

an environment where public domain would contribute to increased restrictions.

Ever tried to get the source for OSX?

It's really quite simple - by allowing the unconditional use of code you are doing just that, allowing it to be used unconditionally. This means someone can take your source code with all the privileges you have granted them, and distribute it without granting others the same privileges.

Now you say "so what?" the original code is still available right? Yes, it is but now improvements to that original code are not available. Furthermore, the original code has now created a proprietary competitor for itself. It will now lose users, developers, and other resources to the proprietary version (with all those fun EULAs). The company that created the proprietary version has effectively hijacked a portion of the development.

The situation gets even worse when you take into account the current state of patent offices in many countries. The open source version may not even be able to implement the same changes.

For more information, please read the excellent philosophy artices on gnu.org.