With sticky poison you can apply poison to a weapon and have it apply across multiple attacks. Thematically it makes sense that if you want to deliver poison via a ranged weapon you need to coat the ammo.
Is there any way to spread 1 dose of poison across multiple pieces of ammo? It would seem to me you could poison a few darts with the same poison it would take to coat a blade.
The way I think about it is to have the poison be applied to one part of the blade, not the whole thing. In this way, it makes sense to only allow ammunition to be dipped once per vial. If you could get three uses out of one poison, no one would poison anything but darts.
I can see both sides of the argument.
One case, one dose is required to get the full effect. So, one dose of poison can not be spread over multiple items because it would have no effect. One does is probably the smallest amount of the poison you can use to produce the effect. It is like potions - you have to use the entire potion to get its effects regardless of your size.
On the other, a fine-sized creatures dagger requires the same amount of poison as a colossal creatures greatsword. That just doesn't sound right.
But in the end, it is nothing more than a balancing factor. Allowing one character to get much more use out of his poison isn't fair to the other poison users just because they use a larger weapon.