Poppy ult vs turrets

Can someone explain to me how Poppy's ult interaction with turrets is fair?

Currently, she ults, and if the turret is targetting her, then the turret will do no damage to her for the duration of Poppy's ult.

This flies in the face of how the turret interacts in all other scenarios. For instance, Yasuo and Braum, will block "all projectiles. . except turret shots;" a similar rule would be fitting for Poppy. Even Fizz, who can dodge a single turret shot, is considered quite OP because of that. Poppy can dodge all turret shots. She can just run in, hit someone, ult them, and then prevent all turret damage while he team goes in; the issue remains even if the defending team falls back to the next turret.

In situations like someone using Zonyas, the turret will change targets. Obviously the definition of Poppy's ult is different from Zonyas, but it seems to me a simple answer would be to have the turret damage either not be blocked, or for the turret to seek out a new target. I just cannot understand how the current situation persists in the game; probably because Poppy is an underutilized champion.