Because people think Teemo counters Darius, and he doesn't, so either the Teemo will win lane and no one will care or he'll lose lane and he'll get yelled at. Lose/lose for Teemo either way. The fact that Darius's grab range is 50 units longer than Teemo's auto attack only furthers my argument.
To beat Darius you just have to be smart... it's actually easy to bait most Darius users into using AND missing Apprehend. But technically if you HAVE to make Darius waste Apprehend all the time then yes Darius counters you. The bigger problem for Teemo is his auto attack sits right on top of Darius' Spins max damage.
Teemo counts as a counter though because of blind and then the heavy poison that out bleeds Darius.
Xbox 360 GamerTag: HolyKnight22 PSN: CrimsonDivide
As a little more in depth justification: to not get grabbed as Teemo when autoing Darius, you MUST be faster than him. You also need to cancel your animation or he will grab you by moving forward. If you can consistently do this without messing up, you might be okay. If Darius is incompetent, you'll be fine. But against relatively even skill, if you get grabbed, you're dead unless he is sub half hp. If he picks up hexdrinker you are screwed.