Tahm Kench's passive

Why does Tahm Kench's passive allow him to both Q stun and then Devour a target without needing to be restacked? Where is the choice in his kit? I suppose you're left with the choice of whether to eat an enemy or eat an ally but it's pretty damn hard to give up that 20 something% hp damage and 2+ second Crowd Control.

And on that same train of thought, why does his passive take so long to drop? Honestly I'm actually fine with the stupidity of the rest of his kit (hello thick skin!) but if I get hit by his Q even once there's no possibility to trade with him without being stunned and eaten because of his Q's naturally low C/D etc.