Obsidian has an entire plugin category for syncing, and recommended alternatives to the official Sync service.
https://community.obsidian.md/search?type=plugin&categories=...
As an Obsidian LiveSync user, there is no way I would recommend using it or other third-party alternatives to my non-technical friends. The initial setup and fly.io setup was worth me saving ~50 Euro/year, but I doubt that holds for my less technically inclined peers.
True, and it's great that they don't block those (they absolutely could).
But those are plugins and aren't as easy to use as the integrated sync. Obsidian wants to have their sync to be the easiest to use, and the easiest to discover.
If they went FOSS anyone could just create a rebranded fork that includes their sync instead of Obsidian's sync. Even GPL wouldn't stop that, if the competitor would just keeps their product open source too.