I think you missed the point with the first comment. It's a show of appreciation, not a charge for a service.

As for your second comment, I think you're mistaken. With the same reasoning, one might think that developers have no incentive to make it easy for users to stop using their application and migrate their data to a competitor's, but this is actually a good thing to do. Google in particular is known for this. I would personally be more inclined to send cake to a site that links me to other good content.

Flattr shouldn't be evaluated purely based on hard, rational economic principles; it's more of a social effect, so it's more complicated than that.