Depends on where marijuana products end up on the regulatory spectrum. Vitamin E acetate is added to thicken the THC + solvent solution, because to users, a thicker solution appears to mean that the THC is less diluted than it actually is.

If marijuana products are regulated like supplements are today, there's really no reason why less than scrupulous vape cartridge manufacturers wouldn't do exactly what they're doing today. There's no regulatory oversight when it comes to supplements.

It's a black-market additive to make their watered down product appear genuine.

It’s an THC product additive, that’s not used in non-THC vape juices. The title is perfectly accurate, even if it could be a little more verbose. I know a lot of commenters here have a vested political interest in promoting marijuana and its derivative products as safe, but you can’t change the facts because of that.

I believe the CDC also found it in illicit nicotine cartridges

The CDC have not found it in any nicotine cartridges. The CDC have also not found the substance in the lungs of any vape users who didn’t use THC products. There are also jurisdictions which have reported finding it in 100% of seized THC cartridges.

It was found in carts that had THC and nicotine combined. That makes it about as much as a nicotine additive as a THC additive, if we are using the incredibly low standards of "found in coexistence with substance sometimes" as the definition of additive.