It is not particularly difficult to mistake a Foongus in tall grass for the top half of a poke ball, try to pick it up, and fall asleep or be left poisoned or paralyzed for one's troubles. This mistake is an easy one, because many of Unova's poke balls use two of these pokemon, instead of a hollowed out Voltorb corpse, as a base. But it is also a deception the Foongus use to their advantage; many wild pokemon approach the Foongus when there is a trainer sufficiently nearby to be mistaken for the "poke ball's" owner, only to become a host for these parasitic pokemon.
The poke ball was invented independently in at least two locations, and the first Foongus looked little like the capsules we know today. They were wholly white in color, and so large that it was impractical to carry them on a belt, or even in most pockets. Early Unovan trainers carried their poke balls in their hand or kept them in their backpack, but the import of Voltorb balls offered far greater convenience.
Wild Foongus had learned a parasitic strategy very soon after they began being harvested for poke balls, for they needed to mature fast enough to survive and reproduce even with the potential of being turned into half of a poke ball. By the time Voltorb balls came to Unova, these Foongus had grown dependent on absorbing nutrients through deception; when their own poke balls were replaced among trainers, wild pokemon no longer mistook Foongus for trainers' balls, so the Foongus were forced to quickly emulate Voltorb's color and size or die trying. This is why Foongus today bear such a strong resemblance to half of a Voltorb, and why poke balls made from the two can not be distinguished in stores.