As others have already pointed out, it strikes me as pretty arbitrary that poison is such a 'bad thing' (I can't help but think of 1066 And All That) in your world.
This is a medieval fantasy world, in which nobody would think of leaving his home without at least a knife. Your generic PC Barbarian typically carries a Greatsword or similar instrument of murder. Your generic PC Wizard typically prepares more offensive spells than anything else. Even the effing Cleric of Ilmater probably packs some heat, maybe a Morningstar, to go with his Full Plate armor.
What difference does poison really make in such a world?
Also, is there a 'defensive' use of such spells as Magic Missile, Scorching Ray or Blindness/Deafness? Is there a 'defensive' use of a Greatsword? Why shouldn't you pack some really nasty critter's venom to defend yourself from armed robbery? Or smear Black Lotus on your (well-wrapped) spellbook because nobody but you has any reason to touch it, ever?
Further, is there any reason mind control magic (spells, Bard songs etc.) wouldn't be banned even harsher? After all, you can subvert an entire society using a bit of charm, suggestion, and domination!
What about a Druid's wildshaping powers? Isn't it just plain impossible to run a state when the dog sleeping by the chancellor's fireplace, or your squire's horse, or a random pigeon on the roof could be a spy?
Summoning and Calling magic? "What do you mean, you can unleash otherworldly horrors at a moment's notice if you so choose? ARREST HIM!"
Resurrection? "What do you mean, just because we executed the Red Reaver and his murderous followers doesn't mean they'll STAY dead?!"
And so on.
Seriously, the only possible reason why poison might be considered a 'bad thing' in your game world, while so many other things are not, is pure, arbitrary, nonsensical social convention. Don't try and come up with a reasonable argument why poison is outlawed - it won't make you look good, all things considered. Instead, you might want to think why so many potentially dangerous things are allowed no questions asked.