I think the reason a lot of the companies won't sell directly is because they fear customers unwittingly making SBRs, getting busted, and then having those customers suing the parent company saying, "Well, RRA, Bushmaster, etc. didn't tell me about NFA laws". I was at a gun show recently, a gentleman was buying a Kel-Tec SU-16 pistol. The pistol already had the barrel shroud, and he was holding it up with his fist balled under it as if her were contemplating adding a forward vertical grip. I asked him, "so, making an AOW are we?" he replied, "What's an AOW". Long story short, I had to explain that what he was going to make would have landed him in jail for 10 years. I know he didn't buy the gun, which made the vendor angry, but I told the vendor that the buyer, after being proscecuted, probably would have sued the vendor for not telling him that he was creating an NFA weapon. This is like what RRA and Bushmaster would face, but on a larger scale.
Most lawyers would probably advise their clients to turn on the seller as being at fault. I'm pretty sure that the AW companies don't want a high-profile case against them. It would probably show up on natoinal TV and bankrupt the company.