Anti-consumer conspiracy theorists may say it was designed to rob the consumer of a small amounts of his/her precious liquid. The truth, however, lies elsewhere.

The concavity is used to provide an air-cushion effect, allowing the factory to move a large amount of cans by blowing air next to the can bases. The cans hover very close to their supporting surface (like a hovercraft). This reduces friction to a point where a lot of cans can be moved with very little effort (normally, by blowing air laterally).

Nifty eh?