The pokeball shaped head and arms of Amoonguss, unlike those of their first form, are useless for creating actual poke balls. The head is far too large to be mistaken for one, despite the similar pattern, so Amoonguss attempt to lure in prey not through their head but their arms. However, most Amoonguss are simply too tall to pull this off; their heads are visible above the grass, and their prey take notice of this fact and do not mistake their identities.
Humans, however, have made use of the Amoonguss by aiding their disguise. Historically, poke balls were kept in a trainer's pocket – hence the name – and it was a common tactic in warfare to place Amoonguss in one pocket and pokeballs in the other, so when enemies tried to knock away a trainer's reserve pokemon, they would often find themselves asleep, paralyzed, or poisoned. This tactic continued in a different form into pokemon battles for sport; an Amoonguss' spores are very difficult to spot, so using these pokemon was an extremely easy way to cheat.
In time, the Amoonguss problem became so severe that Unova's Pokemon League banned putting pokemon in one's pocket outright, and instead required trainers to wear all six balls on their belt, where there is no room for an Amoonguss to hide. Today, increased inspection technology makes this a moot point, and the rule now only applies to Amoonguss trainers, but the convenience of belts has made the name of pockets (or perhaps of pokemon) an anachronism, for few trainers wish to go back to fumbling around a deep pocket to find the right pokeball.