Noticed today that the turrets both heimer & zyra create are not reduced by ninja tabi. The description for the boots states a 10% damage reduction from basic attacks. Both of these champions' turrets deal basic attack damage, albeit ap damage, but they are basic attacks none the less. Similarly, these are also not effected by Jax's counter strike.

Is this by design or a bug?

That's because they're magic damage turrets.

Ninja tabi
+25 Armor UNIQUE Passive: Reduces the damage taken from non-turret basic attacks by 10%. UNIQUE Passive: Enhanced Movement 2 (does not stack with other Boots)

Heimer
H-28G Evolution Turret - Heimerdinger lays down a machine gun turret. Turrets gain abilities as this skill is increased. (Turrets deal half damage to towers).
Heimerdinger constructs a Machine Gun Turret with 275.0/290.0/305.0/320.0/335.0 health and 30/38/46/54/62 (+0.2) magic damage (half damage to turrets). Max: 1/1/2/2/2 turret(s). Heimerdinger stores enough parts for a new turret every 25.0 seconds.

Each rank of this skill grants a further enhancement:
Rank 2: Turret attacks reduce the target's armor and magic resist by 1 (max 50)
Rank 3: Maximum turrets increased to 2
Rank 4: Turrets gain an additional 125 Health
Rank 5: Turret attacks deal area damage
Cost
70/80/90/100/110 Mana
Range
250

Zyra
Rampant Growth: If Deadly Bloom hits a seed, a Thorn Spitter grows, dealing 0.0 (+0.24) magic damage. Lasts 10 seconds.

Rampant Growth: If Grasping Roots hits a seed, a Vine Lasher grows dealing 0.0 (+0.24) magic damage and slowing enemies by 30% for 2 seconds. Lasts 10 seconds.

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So, these turrets do auto attacks. Why does it matter that whether its magic or physical damage?

The utility of ninja tabi is to mitigate the effects of auto attacks, and with these minions/turrets damage source being auto attacks, it should reduce them appropriately.

"Basic attacks" are attacks which apply on-hit effects, or atacks from minions/monsters

Summoned "turrets" like Zyra's plants or heimer turrets are not minions/monsters, nor do they apply on-hit

/thread