Making Zac invulnerable with Teleport

This is a broken mechanic, another players summoner spell shouldn't be able to make a champion invulnerable and thus making his passive a beacon for players to teleport and save their teammate not by their presence but by a mechanic that is abused by players. Recently had a game where the support and top lane both saved their teleport only to abuse this insanely broken mechanic. Honestly made me feel like this needs to be addressed and left me not wanting to continue playing when people can abuse a broken mechanic. It is similar to if a player could teleport onto an Anivia egg to save her from being executed where she otherwise would have been killed. Yes Anivia would come out full health which would make it even more broken but making a champion invulnerable with a summoner spell should NEVER be a thing. Would appreciate some solid feedback and even player insight if you believe this is a fair mechanic.