But many do.
While there's little value right now in "disconfiguring" an AR15 SBR, during the Obama gun panics it was a way to easily sell your SBR for hundreds if not thousands of dollars more. I sold off one SBR lower receiver for considerably more than what I paid for the entire SBR.....and I kept the barreled upper. I sold that barreled upper two years ago for $600.
Original price in 2010 for that Colt 6920 factory SBR.....$800 (used)
Sold complete lower in 2011 for $1200, the barreled upper in 2016 for $600. Net profit $1,0000.
Then again I didn't Dremel my gun either.