Earlier quoted context omitted.
Note that they don't care about the themes their end users pick, though. You can theme your app just fine. They just want operating systems using their software (with their logos, trademarks and support links) to stop shipping their custom themes by default. They just don't want Canonical or Fedora to ship a theme that makes all applications that didn't come bundled look like shit. If you like the Windows 95 Hotdog S…
But end users use the same mechanism to theme their apps as vendors and that one is being removed.
If you have a feature that is used by one customer out of thousands but it's causing problems at every update you push out. It might be better to remove the feature and fire the customer, than to keep supporting it no matter what.