Purify is the one card that will never, ever see the light of day in a tournament. Ever. Unless the tournament has the rule, "You must use poison as your primary source of damage," Purify is the most situational card in the game that can't be used effectively, if at all, because the player using the poison deck will change to another deck 99% of the time, rendering Purify useless. Because everyone knows this, neither player will plan for a poison deck to be used in the following game, so neither player will even think about considering Purify.
The reason a card like this is overpowered is because it does something in addition to removing poison. Because of this, it has many more uses, so it would be used in many more decks sporting water, further decreasing the existence of poison-based decks. If the existence of a single card completely removes the possibility of a deck type, it is overpowered. This is one of those cards.