How does Riot decides if a tank should have hp scaling?

and iirc and

Tahm has it because otherwise he'd be pretty negligible and people would just ignore him, making a melee cc tank with no gap closer or instant cc chain (like leona or nautilus) also deal no damage would be really weak

Braum is make his Q damage scale so it matters after laning phase if only a little bit

And sejuani is kinda hard to justify I think it's to give her something to do while her spells are on CD, And letting her flail scale with hp allows her to function while building towards her primary goal

It's a case by case kinda thing

Tahm has it to make up for his single-target-only CC. It's too high right now, but it makes sense

Braum has it because he has no damage otherwise

Sejuani has to it so that her W feels impactful as a damage spell and to give her late game scaling