In this case there are canonical customers and intel customers.
Intel does not seem to see mir as valuable to their customers, so they do not want to spend time on maintaining support for it. That will change if mir finds lots of users. Intel does provide the source code for their drivers though.
That enables Canonical to providing patched drivers to their customers. They will get the drivers from Canonical/Ubuntu in any case, independent of whether those are patched or not. So they get the same experience either way.
Who exactly is mistreated here?
I do wonder what will happen if AMD or NVidia will do not care to patch their drivers for mir though.