Teemo's Mushrooms

Why is it that Teemo can cover the map in mushrooms? Heimerdinger's turrets deactivate when he leaves. Teemo already has a ton of poke ability , blinds you and causes continuous poison damage. Not too mention the trend is for him to build Liandrys Torment and Ludens Echo. He gets way too OP from then on. His range with his darts is too much as well.