behavior:

splash potions of harming as well as tnt ignited by a player are both able to benefit from random critical hits, and will be buffed by critical hit chance and critical hit damage modifiers on your mainhand item. Also so can bleed damage.

This is not necessarily unintended behavior per se, and i get that the current damage types that cannot crit are ones nessesary for game balancing (percentage health), but it is unintuitive that holding a sword while lighting an explosive would cause the explosive to deal more damage. Also by rpg conventions a critical hit is usually like hitting a weak spot or something, which is not logically possible to do.

That's how it works. I wouldn't call it unintended, just unintuitive. Player attributes are local to the player, not the object supplying the attribute modifier, so once a weapon gives you crit chance you have it for any attack.