> if someone writes an app with a backend service or library, should they be required to document the API for people to build their own tools on top of?

Yes. Network services and client software should be considered two separate products/markets, and tying between them should be seen as anti-competitive behavior worthy of anti-trust enforcement.

That the software industry has been able to develop some brazenly anti-competitive pratices does not constitute an argument that it should be allowed to continue.