My guess is that if the wires run thru the joists, even if people hang stuff from them, they wont cascade fail from overloading like 20+ NM staples would. In VA, crawlspaces are different from basements; with the former, you can perpendicularly run small NM stapled to the bottom side of the joists, because like you say, no one is crawling in crawlspaces and stuffing junk atop the wiring. It's one of the few common sense things we have.
Originally Posted by kwired
The staples for 8/3 - 6/2 or larger are a heck of a lot sturdier than their smaller counterparts; one can just about do a pull up on the wiring and not tear the staple(s) out of the wood.
I'm not sure the delineation point between crawlspace and basement here. A fair number of houses are built on aggressive slopes, where the entry way may have a full-size door and 8' headroom, and on the other side there is barely 2' clearance between the ground and the joists. I havent had to wire one of these, but I would drill everything, run along the band, or use running boards than risk having to pull out a ton of <8ga NM because the AHJ considered the whole shebang a basement.
Electricians do it until it Hertz!