How is this any different from "creatures you control have hexproof"?

The ability implies that you can use this a little like Mother of Runes by activating the ability to target a creature and the hexproof will save them.

Except that wouldn't work at all because the ability would resolve and discontinue the hexproof before the spell attempts to resolve (wherein it would fail because of hexproof)

This could read as: "Whenever a creature you control becomes the target of an ability, it gains hexproof until end of turn".

Now your opponent cannot target them with abilities, and if you have this chick ready to activate then you can give something hexproof in a flash.

That doesn't make the card any more balanced though, both cards are bonkers.