I think its much the same reason many people call the metal boxes that modern firearms use to feed ammo "clips" (which they are NOT--they are "magazines")
A LONG time ago, semi-automatic rifles were actually fed with ammo that WAS held by "clips". The M1 Garand works like this. Because these guns were made by the tens of millions, and used by nearly everyone who served in the military in the US from after WWI until Korea, it became "common knowledge" that semi-auto rifles were fed with "clips". By the mid-1960s, we'd changed over to magazine-fed rifles in our military, but old habits die hard, and anti-gun folks tend to be pretty stupid and don't have any issues playing "fast and loose" with technical terms (or the truth when it comes to ANYTHING about guns), so the usage sort of "stuck"--even though anyone who uses that term for a "magazine" is instantly recognized as an uneducated idiot.
A LONG time ago, this term actually DID apply to anything that wasn't a stick battery, because the only devices that were available that weren't stick batts WERE "mods"--flashlights, laser pointers, mint boxes, whatever--that people MOD-ified to be power sources for using with e-cig cartos and atomizers. Eventually, the manufacturers caught up with the demand for advanced devices, and well, vapers are a pretty lazy lot, and tend to have a lot of techno-geeks in their ranks, and so they tend to like short, cute code words and jargon, so the term "mod" sort of "stuck". Rather than call these devices what they ARE, we continue to call them "mods" because, well, we're basically to lazy to vocalize 3 letters as an acronym (APV) or even TWO syllables (PV).
So why do we call them "mods"--because vapers are lazy... ;-)