"We are not allowed to distribute updates through other channels to apps which were purchased from the App Store."
The solution is simple.
All Mac developers should stop selling through the App Store, regain control of the distribution channel, and leave the Mac App Store barren of quality applications.
Until Mac developers have leverage, Apple will continue taking their 30% cut and enforcing rules which suit them and not the developers.
EDIT:
Just wanted to add...
What are developers going to do when Apple says every single new API in Mavericks can only be used if the app is sold through the Mac App Store? This has already happened with the iCloud SDK, so it's likely they will continue on this path.
What if one day Apple says you can only have a developer certificate if your apps are sold exclusively through the Mac App Store?
The solution is simple.
All Mac developers should stop selling through the App Store, regain control of the distribution channel, and leave the Mac App Store barren of quality applications.
Until Mac developers have leverage, Apple will continue taking their 30% cut and enforcing rules which suit them and not the developers.
EDIT:
Just wanted to add...
What are developers going to do when Apple says every single new API in Mavericks can only be used if the app is sold through the Mac App Store? This has already happened with the iCloud SDK, so it's likely they will continue on this path.
What if one day Apple says you can only have a developer certificate if your apps are sold exclusively through the Mac App Store?