In short, we know that Bethesda/Obsidian like to provide an as-accurate-as-possible simulation of real life America (google a map of Las Vegas or DC and compare it to New Vegas or Fallout 3) and it wouldn't be particularly accurate if there were AK-47s laying about all over the place considering that the importation and purchase of the AK is very strictly monitored in the US. The M16 on the other hand, which is American, is present.
You might want to do some actual research into the matter, that article hardly reflects real world conditions. You bolded out the trivial parts. The reality is that the only thing the AWB achieved is stopping threaded muzzles and conspicuously protruding pistol grips hitting the market for a few years.
Today, all flavors of AKs are still imported as "parts kits" with demilled receivers which are then reassembled on US made receivers, while some models arrive in sporterized configurations and are later converted back to the regular style.
Depending on the state you live in, you're probably able to walk into the nearest pawn shop right now and come out with a semi-automatic AK type rifle without much hassle, if any at all. They're immensely popular and very easy to obtain in the US these days.
EDIT: Oh, there's actually a company based in Vegas that deals with building and converting bulgarian and russian made AKs, so having them in the game would be really authentic.