The sword and the spell are considered honorable because it is something that you weild explicitly and with intent. Poison, on the other hand, is insidious as it takes time to course throught the body and kill.
Example: the dual at the end of hamlet (I can't believe I am useing real reference)
The king and the other dualist both intend to poison hamlet. The king with wine, the dualist by blade. The poison cup kills hamlet's mother by mistake, so the king was evil. The dualist looses the dual to hamlet, and would have died had it been more than a game of sport. Hamlet is doomed, though, because the poison will set in later, after the victory.
After a fireball goes off, that is the end of it and you died or you didn't. In the real world, poision makes you weak or kills you later. I don't consider tear gas or sleep gas (or any chemical that does these things) to be poison because they have temperary, abivilent effects. Any stat drain or death poison, however, has that lasting, nasty effect. Honor means victory or death. If both sides used poison it would be death or weakness.