If you are going to build rifles for yourself without the intention of selling them, you are correct, you get the lower shipped from or thru a ffl, however, if you are buying lowers from ffl's & then building rifles for the purpose of selling them for a profit, that is illegal & the ATF is cracking down hard on people that do this stuff, ie. all the undercover ATF agents at the local gunshows.
The ATF may be scaring people but as of now it is still legal for one individual to sell a rifle to another individual whether they made a profit or not. The lower receiver is still the only registered component on an AR.
You already addressed the accuracy issue? Did you look at the second link with the shooting review for this particular Bushmaster? Granted this is not a top of the line semi auto 308, nor is it a scientific test, but the accuracy in the review shows to be pretty damn good out of the box...the reviewer said that if match ammo & better glass, he could reach SUB MOA. It all depends on what you want to spend.
Sure, 1MOA is good for a short barreled semi-auto. No argument there.
Spend the same money on a bolt action however and you can get in the 1/4MOA range. THAT'S what I consider accurate.
The military is not moving back to the .308 as the primary rifle round. They are starting to issue rifles to Squad Designated Marksman in .308(better to reach out & touch someone in the mountains in Afghanistan). Eventually there will be a 308 or two in each squad. The 5.56 is still the primary weapon caliber, althought they are looking at replacement for the current M855 ammo.
Similar to the "unofficial" use of M1 Garrand's by some over the years instead of the issue AR's...