Actually you don't, according to david pitkin it's the lack of age restriction in the ubuntu store that cause this game to be rejected.

Here's the quote:

Hi Edmund, let's figure the confusion out as soon as possible. There were some crossed wires with the Humble Indie Bundle 5 launch and Super Meat Boy plus the lack of age restrictions in the Ubuntu App Store for Issac but nothing more I thought : (

David Pitkin,Director of Consumer Applications at Canonical.