You miss the point. The same argument could be brought forth against any other commercial license, like MS Office. But you are right, the answer is: none and I consider myself a FOSS advocate, but this is not the world we live in. Obsidian Dev Team puts in work and for them to be able to continue doing so, they need compensation, it's work after all.

Most software doesn't even differentiate between private and commercial use or let's you pay for both, but makes private use cheaper. What obligation does Obsidian suddenly have to be free for commercial use? It's already free for private use, educational purposes and even for freelance work. If someone is making money using a tool, then why is it ridiculous to pay for the tool?