Basically the same people that arm all the other enemies. I figure it's probably "Ada Wong" arming all the zombies and other B.O.W.'s. You have to remember that all of them are controlled and given orders. They don't have the Las Plagas parasite, but apparently they can be controlled. Why else would they not attack "Ada Wong" when she just strolls on by unhindered?
The premise is the c-virus alters the DNA to create a completely different organism. At least that's the case with those that get put into a cocoon state. That could mean that they are also given a set of instructions like arm themselves with dynamite and use a lighter to attack enemies. I'm just making it up, but I think it makes sense, hehe.