But you can sync your Obsidian notes for free if you use free Dropbox or OneDrive accounts.
There's work arounds (for example I forgot the name, but one plugin allows you to setup onedrive to share just one folder via logging in on each device), but the syncing story wasn't great. Quite a bit of extra setup across all devices, more points of failure etc.
Hence the smart business model, of let us handle it for you for a small cost.