I would argue that that represents an unusual case: people here pay you out of respect/gratitude for your contribution to the community.
For unknown authors, the sales picture is different.
For unknown authors, the sales picture is different.
If you don't have a brand or following, though, selling through marketplaces that are not tied to devices is a huge disadvantage.
The typical ipad/kindle/nook user does not go outside the store built-in to the device looking for content (the majority of them probably don't even know it's possible), except for a small, bleeding-edge minority.