Why are some runes allowd to break the rules of champions kits?

Example, tahm kench and phase rush. Becuase of how phase rush is designed tahm can get 100% slow immunity from his own passive slows, this also applies to blitz and his self slows (for clarity, you need boots of swiftness, but even without them, its pretty BS).

Tahm can apply his passive, on the third stack he gains a movemen boost, in which he can then use his W, and then he isnt slowwed, he then carries you to his team, or his tower, or whatever, point is, he can render himself immune ot his own debuff, again this also applies to blitz, and any other melee champion with a self slow debuff.

Why design runes that breaks the rules of a champions kit? Isnt the point to have a rock paper scissors match with champions? To counterpick your opponent? It defeats the purpsoe if you dont have the follow the rules.