This is not about price. Nobody is saying that authors can’t charge for their software. It’s about freedom.

Your computer/smartphone is controlled by the software that runs on it. Libre Software like Jerboa gives you the source code and the right to study it, modify it and distribute it. This means that anyone can verify what the program does and change it if needed. It means that users actually control the program. If the authors added malicious functionality, it would be easy to remove it.

Proprietary software doesn’t give you those freedoms. It’s very difficult to verify what such program does or change it. You can’t really control it, but it controls your device. If it contains malicious functionality, usually you can’t do anything about it. That’s why proprietary software is unethical. It gives developers power over their users and they often abuse that power.