Early in the anime, it's shown Ash pushes the button on the Pokeball before throwing it in order to "activate" it. Simply throwing a Pokeball doesn't do much good since it would simply land wherever it lands and you can pick it back up (but that's never implemented in the games because, well, why would you just throw stuff around and pick it back up?).
When you push the button, however, the Pokeball is in "catch" mode, similar to a hand grenade being armed. When it's then thrown at the Pokemon, its mechanism activates on impact, that's the red miniaturization beam that shrinks the Pokemon and transfers it into the capsule. There seems to be some unexplained internal AI-ish component that locates and identifies the Pokemon, so it doesn't have to be hit by a specific area of the ball (such as the button), although it should technically be possible to throw a locked Pokeball at a target, hope it impacts on the button, thus unlocking it and immediately activating. If it doesn't actually hit the Pokemon, it won't be able to locate it, so the mechanism fizzles. However, since it was already armed, it's "used up" and can't be reverted.
As a whole, the Pokeball is clearly designed based on a hand grenade (and not on "a capsule", as explained by the Professor), and in general, once a hand grenade is armed, its remaining lifetime is very low. Pokeballs are not reusable for the same reasons hand grenades are not reusable.
There's an interesting fact that reinforces this theory: One of the first "modern" hand grenades, prominently used in the American Civil War, was called the "Ketchum grenade".