why can druids use negative armor?

When they use anub and don't have enough armor they still can play a card and it uses health instead of armor as a resource. How is that working as intended. Shouldn't it say use health instead of mana?

By the same logic,the card that removes all the opponents armor should just instagib the enemy hero since hearthstone doesn't differentiate between armor and health...

How about making it function as its worded? Then they would have to work to afford the combo. And not just pull it out of their arse and win despite the state of the game.

Edit : So apparently i was an idiot and saw wrong. Btw, it wouldnt hurt considering people can actually make mistakes.