Yes, well it makes sense that Valve are not going to pay Microsoft 30% of every Steam sale via Steam installs from the Windows store as it'd mean Valve would ge

Yes, well it makes sense that Valve are not going to pay Microsoft 30% of every Steam sale via Steam installs from the Windows store as it'd mean Valve would get 0% of that sale themselves, and they're the ones who built the store and infrastructure up over a many years. Microsoft are just coming in at the last moment and hosting a very small Steam installer binary!