The shields you listed are not free.
Camille has to decide if she needs a AP or AD shield, and that requires her to melee auto someone and stack HP. It is 20% of her max HP. It is great in solo duels, but becomes lackluster in team fights.
Vi is the same way - she has to put herself in danger and expend an ability to trigger her shield. It is 10% of her max HP. She also has to stack HP for it to be relevant. An ability is a cooldown, which is not free.
Riven, like Vi, requires an ability to activate her shield, an ability which she cannot activate if snared. An ability which has a CD. It scales on her AD.
Rumble, like Riven and Vi, requires an ability to activate his shield, which scales on AP, which means he's not getting the most from it if he stacks HP. An ability which has a CD.
Despite my loathing hate for him. I will now defend Yasuo. Yasuo has the closest thing to a free shield. What keeps it from being free is that it requires him to walk to regenerate it and he can't control when it breaks.
Blitzcrank has a shield that shields him for 50% of his current mana. He is actively encouraged to build mana tank items and HP. It's on a long CD, and thus is not free.