Why is lee sin a turret

There are some abilities that do. Lee Sin Q. Corki Q (the bomb thing. I think it's Q). Some others. I can't remember off hand. It was just part of their kit to give true sight. It will probably get reassessed with these changes. They probably wanted to just keep it for now and see if it is too much.

Hard reveal abilities like Lee's E and Morg's R still work.

There are certain abilities that 'reveal' champions. Caitlyn's W and ult, Lulu's E, Nidalee's passive, TF's ult, and so on. They continue to work unchanged from before the stealth update. 'Can't be revealed except by turrets' just means that being near people won't reveal you like it does for Camouflage.