How bizarre. Brand new M16 lowers are like $75 retail. I can't imagine the motive for making them, unless they're just hobbyists who happen to be drug lords.
Here are 18 for sale: http://www.gunbroker.com/Machine-Guns/BI.aspx?Sort=5 (on the first page), ranging in price from $10K to $35K for an unfired Colt AR-15. Plus of course the $200 Federal transfer "tax".
Scare quotes because when it was established in 1934, per the BLS it was a prohibitive $3.7K, or if you don't trust the government's inflation statistics (I don't) 6 oz of gold, which is currently $7.5K.
BTW, if you think government-citizen relations are bad today, well, 15 months before the National Firearms Act was passed, FDR signed an executive order confiscating the gold held by us: https://en.wikipedia.org/wiki/Executive_Order_6102 And it was bought at $20.67/oz, and quickly repriced at $35/oz.
As for crimes, only two, with one by a police officer.
You can, but they're NFA weapons, and require the tax stamp, extra background check, and all that. They're also rather expensive ($15,000ish or more depending on the vintage)--pocket change for cartels, of course, but when you're looking at the limited quantity/availability and high price, it's more cost-effective to make, buy, or steal something sourced cheaper. Plus, nearly all are presently owned by collectors who aren't as likely to let them go except to other collectors unless it goes to auction for whatever reason.
Also, they arm a lot of people. They really do have small armies of soldiers ready to kill, if the videos of them in action prove anything.
More likely though someone just got this $25k milling machine really cheap and decided what the hell. They got busted and they were going to sell to the cartel. News Translation: The Cartel is Making Guns!