>>29823877They block the balls because you throwing it would be cheating. It'd the equivilant of you throwing a rock or something at the pokemon, and as a human, your not supposed to be in the battle. But it won't catch it, even if they didn't block it.
The only exception to date is Ghetis blocking you from catching Kyurem in BW2, with that staff, but that was because Kyurem wasn't caught via pokeball by anyone yet, so it was catchable.
