@Ivafakename evolution takes place in a matter of seconds in Pokémon, so it stands to reason that if people keep leaving pokéballs on the ground it should create a niche that rapid Pokévolution will fill in a short space of time.
This however ignores that disguising yourself as a pokéball conveys no evolutionary advantage, as it attracts trainers who may then capture the Pokémon.
@Ivafakename @CoughingSound also if you think about it wild pokemon will likely stay away from pokeballs because they associate them with being beaned in the head and then captured. So maybe it's a defensive measure
@Nine @Ivafakename very astute, I had overlooked the bean factor entirely