I don't know the US law (I live in Germany), but the intuition is clear:
1. Take some CC0/public domain source code.
2. Build a binary out of it and sell licenses for your software.
Case study: SQLite's source code is public domain. On the other hand, it is embedded in many commercial software products that are sold - and you are not allowed to reverse-engineer the software or redistribute binaries of it. This would be possible/allowed in a no-copyright world.
1. Take some CC0/public domain source code.
2. Build a binary out of it and sell licenses for your software.
Case study: SQLite's source code is public domain. On the other hand, it is embedded in many commercial software products that are sold - and you are not allowed to reverse-engineer the software or redistribute binaries of it. This would be possible/allowed in a no-copyright world.